Ever had a tire question? We’ve got all the answers!
1. Airtight Synthetic Rubber Liner
Most modern tubeless tires are constructed with a virtually impermeable butyl rubber liner. This liner replaces the old inner tubes. Check your tire’s air pressure monthly, as some air loss occurs over time.
2. Carcass Ply
The carcass ply is comprised of thin textile fiber cables that are bonded into the rubber. These fiber cables are largely responsible for determining the strength of the tire.
The beads are responsible for clamping the tire firmly against the rim of the wheel.
In addition to giving the tire its height, the sidewall protects the tire against impacts with curbs and other objects. The sidewall also contains all the markings which tell you the important information about the tire, such as speed rating, load rating, and tire dimensions.
5. Crown Plies
Crown plies provide the rigid base for the tread which allows for good gas mileage. The plies also provide centrifugal and lateral rigidity to the tire, while also allowing the tire to flex sufficiently for a comfortable ride.
The tread is designed to provide traction in a variety of conditions. Good tread design also resists wear, abrasion, and heat.
Practically everything you need to know about a tire can be found on its sidewall.
The following pieces of information are typically branded on the sidewall of the tire:
- Tire Manufacturer
- Tire Name
- Tire Size (Width, Aspect Ratio, Rim Diameter)
- Load Index & Speed Symbol
- U.S. DOT Tire Identification Number
- Severe Conditions (Snow, Mud)
- Tire Ply Composition and Materials
- Maximum Load Rating
- Treadwear, Traction and Temperature Grades
- Maximum Permissible Inflation Pressure
1. Tread is worn down in the center of the tire
If the tread is worn down in the center of the tire, it may mean that the tire has been used while overinflated for a significant period of time. This issue is easy to prevent – simply check the recommended air pressure for the tire, and adjust accordingly.
2. Tread is worn on the inside and outside shoulders of the tire
If the tread is worn down on both shoulders of the tire, it may mean that the tire has been used while underinflated for a significant period of time. Again, the solution is to check the recommended air pressure and adjust as needed.
3. Tread is worn on the inside or outside shoulder of the tire
Tread wear on either the inside or outside shoulder of the tire may indicate that the wheel is out of alignment. We recommend taking your vehicle to one of our certified installers for a professional wheel alignment.
4. Tread is worn to the left or right of center
There are two main issues that can cause the tread to be worn to the left or right of the center — the tire may be out of balance, or there may be a problem with the vehicles steering or suspension. This condition is called “cupping” and should be referred to a professional mechanic for service.
Measuring your tire tread depth with a penny is a simple task.
- Step 1: Pinch the penny so that the words “In God We Trust” and the top of Lincoln’s head are showing
- Step 2: Place the top of Lincoln’s head into one of the tire tread grooves
If you can see above Lincoln’s head or any of the “In God We Trust” letters, then your tire tread is too low. You should also repeat the steps in other grooves of the tire tread, because a tire can wear differently in various spots.
Measuring your tire tread with a Tire Depth Gauge is more expensive than the penny test, but there are many tire gauges on the market that are very inexpensive, and they are also more accurate. To use the Tire Depth Gauge, simply follow its owner’s manual. Though the process will vary by gauge, none should be much more complicated than the penny test. The Tire Depth Gauge will give you an immediate, accurate measurement of the tire tread depth for the entire tire, so there is no need to measure multiple spots.
Here are some other actions you can take to properly maintain your tire tread depth:
- Measure tire tread depth every month
- Maintain the tire pressure that the manufacturer recommends
- Rotate your tires every 5,000 miles
- Visually check your tires for damage or wear
Service type ratings
Most tire sizes begin with one or more letters — for example, T or P. The letter tells us what type of vehicle or service the tire was designed for.
T = Temporary Spare (Example: T145/70R17 106M)
If the tire size begins with a T, it means that the tire is a temporary spare. Also known as mini spares or space savers, temporary spares are designed for short-term use until the regular tire is repaired or replaced.
P = P-Metric (Example: P215/65R17 98T)
P-Metric tires are the most common type of tire. The P stands for passenger vehicle, meaning that these tires are designed for use on passenger vehicles like cars, minivans, light-duty pickup trucks (¼ ton or ½ ton capacity), and SUVs.
Metric / Euro-Metric (Example: 185/65R15 88T)
Metric tires, also known as Euro-Metric tires because the sizing originated in Europe, don’t have a letter designation. Euro-Metric sizes are equivalent to P-Metric sizes in dimensions, but Euro-Metric sizes have subtle differences in their load-carrying ratings and capabilities. Euro-Metric tires are usually found on European cars, but they’re also used frequently used on SUVs and vans.
LT – Light Truck (prefix) (Example: LT235/75R15 104/101S/C)
Light Truck Metric tires sizes begin with the letters LT. These tires are designed for use on vehicles used to tow trailers or carry heavy loads. This includes SUVs, full-size vans, and medium-duty and heavy-duty pickup trucks with ¾ ton to 1-ton capacity.
LT – Light Truck (suffix) (Example: 9.5-16.5LT 121/117R)
These tires are made for light-duty, medium-duty, and heavy-duty pickup trucks (typically ½ ton, ¾ ton or 1-ton load capacity), sport utility vehicles, and vans.
ST – Special Trailer (Example: ST175/80R13)
Tires beginning with ST are special trailer tires and should only be used on car, boat, or utility trailers.
C = Commercial (Example: 31×10.50R15/C 109R)
Euro-Metric tire sizes ending with a C are commercial tires, for use on delivery trucks and vans capable of carrying heavy loads. In addition to the C designation, these sizes are also branded with a load range and service description rating (load range B, C, or D).
Other ratings and measurements
Section width of tire (Example: P225/45R17 91V)
The three digits following the service type prefix (if present) tell us the cross-sectional width of the tire in millimeters.
In the example above, the tires width, measured from the widest point of the inner sidewall to the widest point of the outer sidewall when properly mounted, is 225 millimeters. The section width can be converted to inches by dividing the width in millimeters by 25.4 like so: (225 millimeters) / (25.4 mm/in) = 8.86 inches.
Aspect ratio of sidewall (Example: P225/45R17 91V)
The two-digit number that usually follows the tire’s section width tells us the aspect ratio, or tire profile measurement.
In this example, the 45 indicates that the sidewall distance, from the wheel rim to the outside of the tread, is 45% of the section width. A lower aspect ratio means a lower-profile tire with a shorter sidewall, while a tire with a higher aspect ratio will have a taller sidewall and look more like a donut. Because we know that the tire size shown in this example has a section width of 8.86 inches and the aspect ratio is 45%, the sidewall height for this tire is 3.98 inches: (8.86 inches) x (.45) = 3.98 inches.
Tire and wheel diameter (Example: P225/45R17 91V)
Again using our example tire size from above, the 17 means that the tire should be matched to a 17-inch diameter wheel.
Tires usually come in the following widths (in inches): 8, 10, 12, 13, 14, 15, 17, 18, 19, 20, 22, 23, 24, 26, and 28. Tires in these sizes are typically found on most passenger cars, light-duty light trucks, SUVs, minivans, and vans. Tires with a rim diameter measured in inches are called “inch rim” sizes.
Unique wheel diameters
In addition to the inch rim sizes, there are also some unique tire sizes out there. Although not as common, tires are made in half-inch diameters for some heavy-duty light trucks, box vans, and heavy-duty trailers. These sizes are usually 14.5, 15.5, 16.5, 17.5, and 19.5 inches, and an example would be 33×12.5R16.5 118R.
The diameter of tires and wheels can also be measured in millimeters. These millimetric sizes include 365, 390, and 415. Michelin originally introduced millimetric sizes in the 1970s and 1980s with their TRX tires. In North America, Michelin also introduced run-flat tires (integrated tire & wheel combinations) called the Michelin PAX System on a limited basis as OE (original equipment) on some cars. For example, a PAX System size of 245-680R460A 102V expresses tire and wheel dimensions as follows:
The section width is 245 millimeters
The overall tire diameter is 680 millimeters
The rim diameter is 460 millimeters
Asymmetric tire beads
In the example above, the A in 460A tells us that these tires have asymmetric beads. The outside bead (450 mm) and inside bead (470 mm) are different diameters. Each of the unique diameter sizes for tires and wheels were developed to address some specific need in the industry, because either the tire/wheel design or the intended vehicle use dictated the unique sizing.
The uniquely sized tire diameters have bead profiles that are different shapes than traditional tire beads. As a result, tires and wheels with unique rim diameters should never be combined with traditional inch rim tires and wheels. Before mounting tires on wheels, the tire and wheel diameters should always be confirmed to match.
Internal construction (Example: P225/45R17 91V)
When a letter (B, D, or R) follows the two-digit aspect ratio, it tells us the tires construction. In this example, the R means that the tire has radial construction. Over 98% of all tires sold today are radial tires, where the internal body plies of the tires construction radiate outward from the center.
If there’s a D instead of an R, the tire has a bias ply construction, where the internal body plies of the tire crisscross on a diagonal pattern. In belted tires (marked as B), the internal plies crisscross like in a D construction, but there’s also an extra layer of reinforcing belts under the tread area. Belted tires are rarely seen these days.
Speed rating (Example: P225/45ZR18 92Y)
Today, the only speed rating still included in the tire size is the Z rating (sports cars). Since 1991, all other speed ratings are included in the service description.
Service description rating (Example: P225/45R17 91V)
Since 1991, the service description rating is mandatory for all speed ratings (except Z-rated tires) and appears at the end of the tires size brand. The service description is used to identify the tires load index and speed rating.
With the exception of Z-Rated tires, all speed-rated tires manufactured since 1991 require a Service Description code. The first 2-3 digits of the code is a numerical value representing the tire’s Load Rating, followed by a single character that represents the tire’s Speed Rating.
In our example above, the load index of “91” means that the tire can carry approximately 1,356 pounds. As you can see in the following table, the higher the tire’s Load Rating, the greater the load it can carry.
|Load Index||Pounds (lbs)||Load Index||Pounds (lbs)|
Speed Ratings were established so that tires could be correctly matched to the top speeds of vehicles. If you happen to be driving on the Autobahn in Germany, there are no speed limits and driving at high speeds is legal. However, in the United States, we are legally bound by designated speed limits, so it is unlikely that you will ever achieve top speeds for your tires. Manufacturers also do not recommend using their tires in excess of the legal limits, even though the tires are engineered and constructed to be capable of high speeds.
The Speed Rating for tires is derived based on laboratory tests. Tires are mounted in contact with a large drum that simulates driving load and the speed is increased by 6.2 mph increments every 10 minutes until the maximum speed is reached.
Speed Ratings only apply to tires that have never been damaged, cut, punctured, under-inflated, over-inflated, or altered in any other way. Most tire manufacturers maintain that the tire’s Speed Rating is no longer valid if the tire has been repaired due to a cut or puncture because the quality of the repair cannot be controlled or verified by the manufacturer.
The Speed Rating is a single character code that describes the maximum speed of the tire:
|N||87 mph||Temporary Spare Tires|
|Q||99 mph||Studless and Studded Winter Tires|
|R||106 mph||Heavy Duty Light Truck Tires|
|S||112 mph||Family Sedans & Vans|
|T||118 mph||Family Sedans & Vans|
|H||130 mph||Sport Sedans & Coupes|
|V||149 mph||Sport Sedans, Coupes & Sports Cars|
|W||168 mph||Exotic Sports Cars|
|Y||186 mph||Exotic Sports Cars|
|Z||149+ mph||Sports Cars|
Over the years, the Speed Rating symbols were marked on tires in a variety of ways. Many early tires had the Speed Rating included within the tire sizing information. For example:
However, since 1991, the speed symbol has been required to be shown only in the Speed Rating portion of the tire’s Service Description (example 220/45R17 91V) to denote the tire’s fixed maximum speed capability. The exception to this rule occurs with Z-rated tires. The Z Speed Rating often appears within the tire size (example 220/45ZR17) and signifies a maximum speed in excess of 149 mph.
When Z-rated tires were first introduced, it was thought that the maximum speed of 149+ mph would be sufficient to describe the top speeds of the tires. However, it later became apparent that the Z-rated tires did not specify an upper bound beyond 149 mph. Hence, in order to accommodate new vehicles with extremely high speed capabilities, the automotive industry created the W and Y ratings to specify the top speeds of 168 mph and 186 mph, respectively.
Most recently, Z-rated tires may also show a W or Y rating in the Service Description. For example:
- 220/45ZR17 90W
- 220/45ZR17 94Y
- 220/40ZR18 (92Y)
In the first example above, the Z-rating indicates that the tire is rated for 149+ mph. In the second example, the 90W indicates that the tire is rated for 149+ mph with a maximum speed rating of 168 mph. Similarly, the 94Y in the third example indicates a tire rated for 149+ mph with a maximum speed of 186 mph. In the fourth example, the (92Y) enclosed in parentheses indicates that the tire has been tested at speeds in excess of 186 mph.
Why is it important?
Rotating your tires is like exercise. It’s one of those things that everyone knows they should do – but they still might not always do it. Here’s why you should rotate your tires every 5,000 miles:
- Helps maintain your vehicle’s handling and safety
- Helps the tires wear more evenly, so they last longer
- Many tire warranties require tire rotation to keep the warranty valid
How tires wear
As you drive your vehicle, your tires will inevitably wear. The placement and extent of the wear depends on a few factors, like the tire’s position on the vehicle. Each axle has different external forces to deal with – for example, in a front-wheel drive vehicle, the front tires are exposed to more intense forces than the back tires. So the front tires on a front-wheel drive car will show more wear than the front tires on a rear-wheel or all-wheel drive vehicle.
If your tires wear unevenly, you may eventually need to replace one or two while the others are still perfectly fine. Uneven tire wear can also affect the stability and handling of your vehicle. If your tires are unevenly worn, they may not respond as quickly and this can jeopardize your vehicle’s safety.
How tire rotation can help
Rotating your tires regularly helps to evenly distribute tire wear. This will prevent one or two of your tires from becoming significantly more worn than the others – so ultimately, it prolongs the lives of all your tires. Tire rotation can also help keep your tire warranty valid, because many manufacturers require you to rotate your tires.
For the best results, you should rotate your tires every 5,000 miles. To make it easy to remember, just have them rotated every other time you get your oil changed.
If your tires are showing severe signs of wear, you should have us check them for mechanical issues. If no mechanical issues can be found, the wear may simply be a result of your driving style. In this case you may want to have your tires rotated more frequently than the standard 5,000 miles.
- If your tires are “directional” or “non-directional”
- If your vehicle is Front-Wheel-Drive (FWD) or Rear-Wheel-Drive (RWD)
- If your vehicle is Four-Wheel-Drive (4WD) or All-Wheel-Drive (AWD)
- If you have a full-size spare tire or a mini-spare
- If you have staggered fitment tires (different sizes and/or offsets on front and rear)
Non-directional tires, same size & offset on front & rear
The Tire & Rim Association has identified three main rotation patterns that can be used for most vehicles in this scenario.
1. Forward Cross
Use this pattern if your vehicle is a FWD vehicle. Move the front tires straight back to the rear, and bring the rear tires to the opposite side of the front axle.
If your vehicle is a FWD vehicle, you can use this as alternate pattern instead of the #1 Forward Cross. Move the front tires to the opposite rear positions and move the rear tires to the opposite front positions.
3. Rearward Cross
Use this pattern if your vehicle is a RWD or 4WD vehicle. Move the rear tires straight up to the front and move the front tires to the opposite rear positions. Directional and/or staggered high-performance tires & wheels in addition to the three main rotation patterns listed above, there are two additional patterns that can be used for today’s high-performance tire and wheel trends.
Directional and/or staggered high-performance tires & wheels
In addition to the three main rotation patterns listed above, there are two additional patterns that can be used for todays high-performance tire and wheels.
Directional tires are designed and constructed so that they always rotate in the same direction due to their tread pattern. If your tires are directional, they should only be rotated from front-to-back (or vice-versa) on the same side of the vehicle. If you have the same size and offset directional tires, use Pattern #4. Move the front tires to the rear on the same side and the rear tires to the front axle on the same side of the vehicle.
If you have differently-sized non-directional tires on the front and rear axles, use the side-to-side Pattern #5. Move the front tires to the opposite side of the front axle and the rear tires to the opposite rear side. If you have differently-sized directional tires on the front and rear, you cannot use Pattern #5 as shown. Instead, you will need to dismount the tires from the wheels, and remount and balance the tires on the opposite wheels.
Five tire rotation – full size spare tire
If your vehicle is equipped with a temporary or “mini-spare” tire, it cannot be included in your tire rotation practice. However, if your vehicle is equipped with a full size, non-directional spare tire (not branded “for temporary use”) and wheel the same size as your four main tires & wheels, you may want to consider the following rotation patterns to keep all five tires evenly worn. This is especially important for 4WD and AWD vehicles, because if you need to use your spare tire and it has different wear than your other three tires, it could place undue forces on the drive train of your vehicle due to the difference in tread depth.
First, check your vehicle’s owner’s manual to see if they have a recommended tire rotation pattern and follow those guidelines. If there is no rotation information available, consider the following patterns:
6. Forward Cross for Front-Wheel-Drive
If your vehicle is FWD and you have non-directional tires with a full-size matching spare, use Pattern #6. Move the rear tires to the opposite front positions. Move the left front tire to the left rear position. Move the spare tire to the right rear position and let the right front tire become the spare.
7. Rearward Cross for Rear-Wheel-Drive or 4-Wheel-Drive
If your vehicle is RWD or 4WD with non-directional tires and a full-size matching spare, use Pattern #7. Move the left rear tire to the left front position. Move the right rear tire to the right front position. Move the right front tire to the left rear position. Move the spare tire to the right rear position and let the left front tire become the spare.
While the tire’s sidewall does identify a maximum cold inflation pressure, it is not necessarily the correct pressure to use for your vehicle. Your vehicle load might be different than that of other vehicles that use the same tire, so the tire pressure listed on the sidewall could be inaccurate for your specific vehicle. In fact, almost all vehicle manufacturers specify a recommended tire inflation pressure that is less than the tire pressure listed on the tire’s sidewall.
You should check the manufacturer’s tire inflation recommendation and use that as the basis for your inflation pressure. So where do you find the manufacturer’s recommendation?
First, check the vehicle’s owner’s manual, because many manufacturers list the recommended tire pressure there. In some cases, the manufacturer may also specify alternate pressures based on load and/or speed. In fact, new vehicle owner’s manuals must address these five subjects:
- Tire Labeling
- Tire Inflation Pressure
- Glossary of Tire Terms
- Tire Care
- Vehicle Load Limits
Regarding Vehicle Load Limits, manufacturers are required to identify the vehicle’s load capacity with the following sentence: “The combined weight of occupants and cargo should never exceed XXXX pounds.” For safety, it is extremely important to stay below the maximum vehicle load rating. For example, it would be quite possible to exceed a vehicle’s load capacity if you were to carry additional passengers and extra cargo, such as luggage, in a rooftop carrier and/or the trunk space.
In addition to the owner’s manual, you should also be able to find tire inflation information on the vehicle’s tire information placard. Beginning in 2003, vehicle manufacturers were directed to standardize the format and placement of tire information placards. The tire information placards identify the vehicle weight capacity, the OE (Original Equipment) tire sizes (including the spare tire) and the recommended inflation pressures for the tires. On new vehicles, the placard must be located on the “B-pillar”, which is the driver’s side doorjamb. For vehicles that do not have a B-pillar, the placard should be attached to the rear edge of the driver’s door, unless it is too narrow, in which case it should be placed on an inward-facing surface next to the driver’s seat. On older vehicles, placards can also be found in other locations inside the vehicle:
- The driver-side door or doorjamb
- Rear passenger doorjamb of Ford sedans
- Fuel filler door
- Glove box or center console door
- The engine compartment
Once you have located the recommended cold tire inflation pressure and vehicle load in either the owner’s manual or on the tire information placard, the next thing to understand is how temperature can affect the inflation reading. For complete details, see our article Tire Inflation Pressure and Temperature. For now, suffice it to say that the tire inflation pressure should be checked first thing in the morning when the tires are at their coldest- before the vehicle has been driven, before the tires have been in direct sunlight, and before the temperature rises.
Also, you need to use a proper quality tire gauge to measure the inflation pressure. Do not rely on a visual check because it is not possible to accurately measure tire inflation pressure with only a visual inspection.
In summary, all tires, regardless of size, will experience a decrease in the designed performance (load capacity, traction, handling, and durability) if the tire inflation pressure is either too low or too high. Make sure you achieve optimum performance and safety of your tires by maintaining the proper recommended tire inflation pressure(s).
First, it can be very difficult to tell if a tire is under-inflated or over-inflated by simply looking at it. The only way to truly assess the tire’s air pressure is to correctly measure it with a proper quality tire gauge. Always check your tires’ air pressures at least once a month, and prior to taking any highway trips. If necessary, adjust your air pressures according to the vehicle’s owners manual or as indicated on the vehicle tire placard in the door frame.
Second, the tire’s load capacity is determined by tire size and inflation pressure. Smaller tires have lower load capacities than larger tires, just as tires with low air pressure have lower load capacities than tires with high air pressure. Again, make sure your tires are inflated to the proper inflation pressure for the load you’re carrying. Also, be sure not to overload your vehicle and exceed the load rating of your tires.
Third, when a tire is under-inflated or over-inflated, the tire can (and probably will) wear unevenly. Proper air pressure in the tire will provide an even distribution of load across the tread pattern and will optimize the stability of the tire’s structure. Under-inflated tires will tend to wear faster on the outside edges of the tread pattern, whereas over-inflated tires will tend to wear faster down the middle of the tread pattern. Under-inflated tires will also experience higher rolling resistance and decreased durability.
Finally, under-inflated tires will provide less stability for the vehicle. This is because the tire will flex more when cornering and stopping due to it having less air pressure than needed. Under-inflated tires will not respond as quickly as they would when properly inflated, thus negating the performance that was originally designed by the tire manufacturer.
Tire pressure can change with fluctuations in temperature. So, how should you correctly monitor tire pressure as the outside temperature changes?
First, it is important to remember that gas expands when heated and contracts when the temperature declines. In North America, the daily temperatures rise and fall between day and night, as well as seasonally. As the days get shorter and colder during Fall and Winter, it is especially important to check tire pressure.
Second, it is important to know that the recommended tire pressure for your vehicle (as specified in the owners manual) and the tire placard for the vehicle are both based on cold inflation pressure. This means that the tire pressure should be checked in the morning before the tire has been run, before the ambient temperature rises during the day, and before the tire is exposed to direct sunlight.
A good estimate to use when comparing tire pressure to air temperature is for every 10°F, tire pressure will adjust by 1 psi. For example, if the outside air temperature increases 10 degrees, the tire pressure will increase by 1 psi. Conversely, if the air temperature falls 10 degrees, the tire pressure will decrease by 1 psi.
In most parts of North America, the difference between average summer temperatures and average winter temperatures is about 50°F. This means that your tires will fluctuate approximately 5 psi (assuming no other air loss) between the coldest and warmest times of the year. A drop of 5 psi during colder months will affect traction, handling, and durability. This is why it is important to remember to check your inflation pressure, especially during colder times of the year.
In most parts of North America, the average daily air temperature fluctuates by approximately 20°F. Using our rule-of-thumb formula from above, this means that the inflation pressure can fluctuate by approximately 2 psi during the day. This fluctuation can be even more pronounced if the tire is subject to direct sunlight.
In addition to changes from sunlight, the tire’s temperature is also affected by driving. Tire pressures can increase by up to 5 psi in the first 20-30 minutes of driving, before finally stabilizing.
If you live in a cold climate and have a heated garage, the opposite problem can occur. As soon as you leave the confines of your heated garage and go out into the colder weather, your tires can lose inflation pressure. Again, for every 10°F dropped, the tires will lose approximately 1 psi each.
In all three scenarios, the same problem arises, regardless if you check the air pressure when the tires are warm from driving, when there are ambient temperatures, or if the tires are in direct sunlight. If the inflation pressure is correct when a tire is warm, then it will likely be anywhere from 2-5 psi under-inflated when it is cold.
Bottom line- you should always check your inflation pressure when your tires are cold, and make sure to inflate them to the recommended pressure at that time.
Tire pressure changes over time. Think of how balloons begin to lose air after a few days…the same thing occurs in tires. The air pressure inside the tire causes very small amounts of air to escape through the walls of the tire over time. Even though the tires are fairly airtight, there are still microscopic pores in the tire materials that let air out. This effect is called permeation.
Generally speaking, assuming no other air loss (such as a puncture or pin-hole leak), a tire inflated between 30-35 psi will lose approximately 1 psi per month. This means that if you neglect to add air and maintain proper inflation pressure for a few months, your tires could be 2-3 psi under-inflated.
The bottom line is that tire inflation pressure should be checked monthly. This will help to ensure that your tires are achieving their optimal traction, stability, and durability. In addition, by checking your tire inflation pressure monthly, you may notice if you have additional leakage due to a puncture or other small leak. It is much better to detect a problem early before the tire is damaged due to excessively low pressure.
The U.S. Department of Transportation (DOT) National Highway Traffic Safety Administration (NHTSA) requires the DOT number to be banded on the tire’s sidewall. This number begins with the letters “DOT” and is followed by a series of 10-12 characters. The additional characters identify the manufacturing location, the tire size, the manufacturer’s code, and the week and year the tire was manufactured. Therefore, it is easy to determine the age of the tire by looking at the DOT code.
If your tires were manufactured before the year 2000, the date of manufacture will be the last 3 digits of the DOT code. This is because tires were assumed to have a life of ten years or less, so the last digit is actually the year within the current decade. The first two digits refer to the week within that year. For example, if the last 3 digits were “022”, it would mean that the tire was manufactured in the second week of the year, and the year is the second year of the decade. In this example, the tire could have been manufactured in 1982 or 1992. While there is no universal identifier used to signify which decade, some tires have a small triangle following the DOT code to indicate the 1990’s.
Beginning in the year 2000, all tires manufactured have had a 4-digit code which uses 2 digits for the year and 2 digits for the week of the year during which the tire was manufactured. For example, if the last four digits of the DOT code were “0203”, it would signify that the tire was manufactured during the second week of the year 2003.
If you look at the tire’s sidewall and see a DOT number that appears to be incomplete, that is because the current regulations require the entire DOT number to be branded on only one sidewall, while the opposite sidewall is branded with just the first few digits. To see the entire DOT number, simply look on the other sidewall.
There is one other way to determine the age of your tires – by keeping your sales receipt! It’s a good idea to hang on to your sales receipt in case you have a warranty claim for the tire(s). Most warranties from tire manufacturers cover the tires for four years from the date of purchase, or five years from the week that the tires were manufactured. If you lose your sales receipt, you could actually limit your warranty coverage. For example, if you purchased new tires today that were manufactured two years ago, your warranty would last for 4 years from today’s date since that is the date of purchase. However, if you lost your sales receipt, then the tires would be covered for five years from the date of manufacture, which was two years ago. Therefore, your warranty would end three years from now, not four, meaning you would lose a year of coverage.
According to current regulations, the DOT Tire Identification Number must begin with the letters “DOT” followed by a series of characters (usually 10, 11, or 12 numbers, letters, or symbols). These characters identify the tire size and manufacturer’s specifications, as well as the manufacturing location and date the tire was manufactured (listed as week and year). Many people mistakenly refer to the DOT number as a product serial number, but the DOT number is not actually a serial number – rather, it is the identification of the manufacturer’s production batches.
If you look at the tire’s sidewall and see a DOT number that appears to be incomplete, it is because the current regulations require the entire DOT number to be branded on only one sidewall, while the opposite sidewall is branded with just the first few digits. To see the entire DOT number, simply look on the other sidewall.
If your tire does not have a DOT number branded on its side, then the tire does not meet all U.S. requirements. Not all tires are intended to be imported and sold in the United States, so the tire in question may have been developed and produced without being tested and confirmed by its manufacturer that it has met all U.S. Federal Vehicle Safety Standards (FMVSS). As a result, these tires cannot be legally driven in the United States.
When you purchase new tires, you should record the DOT numbers of your tires and send them to the manufacturer. This information is critical in the event of a tire safety recall. You may be able to register online on the manufacturer’s website.
The contact patch will vary in shape and size depending on the geometry of the tire, and this affects various performance characteristics of the tire. Tires with a higher profile or aspect ratio (that is, the height of the sidewall is fairly large compared to the section width of the tire), tend to have a long and narrow contact patch. Typically, these would be passenger cars and light truck tires. The longer, narrower contact patch gives these tires a smooth ride and allows them to handle in a predictable manner. They may also have especially good traction in snowy conditions.
Conversely, most high-performance and ultra-high performance tires have a lower profile (lower aspect ratio). As a result, the contact patch is generally shorter and wider, which makes them very effective in cornering stability and traction. This also allows the high-performance and ultra-high performance vehicles to have very responsive handling, especially on dry roads.
First, it is a good idea to understand the difference between OE (Original Equipment) tires that are installed on a new vehicle versus replacement tires. In the United States, vehicle manufacturers are mandated to comply with certain average fuel economy standards for their entire product line or “fleet” of vehicles. For example, the Corporate Average Fuel Economy (CAFE), that is mandated by the government for the current year is 27.5 miles per gallon for cars and 20.7 miles per gallon for light trucks, minivans, vans, and sport utility vehicles. Because the CAFE is an average fuel economy, vehicle manufacturers must sell both small and large vehicles that have varying fuel economies in order to meet the average fuel economy amounts for their entire product line. If the manufacturer’s fleet sales have a better fuel economy than the mandated average, the vehicle manufacturers can earn “credits” towards future years. Conversely, if the average fuel economy falls below the mandated average, the vehicle manufacturer can be fined.
As a result, manufacturers look for ways to improve the fuel economy of their vehicles. Fuel economy is the result of overcoming restrictive forces on the vehicle. These forces include:
- Wind Drag / Aerodynamics
- Vehicle Weight
- Rolling Resistance
- Driveline Friction
- Road Grade / Hills
- Stopping / Starting (overcoming Newton’s Law of Inertia)
In other words, to achieve the same overall performance, heavier vehicles will require more fuel than lighter vehicles, especially on inclines. Generally speaking, vehicles with a “boxier” design will require more fuel than sleek, aerodynamic vehicles. Stopping and starting in city driving will require more fuel than steady driving on the highway. And finally, the rolling resistance of tires can affect fuel economy.
Because the CAFE standards are so important to vehicle manufacturers, many of the manufacturers demand that the tire manufacturers develop tires with low rolling resistance to be used as Original Equipment on new vehicles. Consequently, OE tires may be constructed with slightly thinner sidewalls and shallower tread depths, as well as special materials and tread compounds in order to decrease weight and improve rolling resistance.
The question is, how much does the rolling resistance of tires affect the overall fuel economy? First, understand that the CAFE testing is performed in a laboratory under ideal conditions. The real-world fuel economy is affected by the factors described above, as well as the driving conditions.
For example, during city driving with frequent stops and starts, it is estimated that one of the greatest factors affecting fuel economy is overcoming inertia, which is responsible for about 35% of the vehicle’s resistance. Driveline friction accounts for about 45% of the vehicle’s resistance, whereas air drag is relatively negligible at about 5% due to low speeds. That leaves about 15% for the tire rolling resistance.
Contrast city driving to steady highway driving. In this case, overcoming inertia from starting and stopping is practically negligible. Instead, the greatest resistance force is air drag, which accounts for about 60% of the resistive force on the vehicle. Driveline friction is about 15% and tire rolling resistance is closer to 25%.
To translate rolling resistance into a more tangible value, let’s consider the following example. Assume you are purchasing a new set of tires to replace your OE tires. Let’s further assume that the new tires are high performance tires that increase rolling resistance by about 18-20% compared to the OE tires. According to our estimates above, the rolling resistance of tires has a different effect on fuel economy in city driving compared to highway driving. In order to calculate the difference, we need to multiply the 18-20% difference in the tires by the amount of influence in the vehicle’s overall resistive forces:
City Driving = (15% Rolling Resistance Amount) x (18% to 20% Increase) = 2.7% to 3.0% Highway Driving = (25% Rolling Resistance Amount) x (18% to 20% Increase) = 4.5% to 5.0%
Assume your vehicle normally gets 24 miles/gallon in city driving and 31 miles/gallon for highway driving. With the new set of tires, your fuel economy would be affected as follows:
City: (24 mpg) x (2.7% to 3.0%) = reduction of 0.65 mpg to 0.72 mpg Highway: (31 mpg) x (4.5% to 5.0%) = reduction of 1.40 mpg to 1.55 mpg
Clearly, the effect on city driving is not too great, but highway driving is affected substantially. If you drove 20,000 miles per year and 50% of it was highway driving, and assuming average gasoline costs of $3.75 per gallon, in this example you would use nearly $110 more in fuel per year simply due to the tires. The costs are magnified as fuel prices continue to increase.
It must also be mentioned that one of the best and easiest ways to improve your rolling resistance (or at least minimize it) is to keep your tires properly inflated to the correct tire pressure. Under-inflated tires can experience dramatic increases in rolling resistance. For example, if your vehicle recommends that the tires be inflated to 35 psi and the tires are actually under-inflated at 29 psi, they will experience an increase in rolling resistance of approximately 12%.
In summary, not only is it important to choose tires with good rolling resistance characteristics, it is equally important to keep your tires properly inflated to the vehicle manufacturer’s recommended air pressure.
The primary reason you need to break-in your tires is because during manufacturing they are coated with a release lubricant. This happens during the curing process to prevent them from sticking to their moulds. Some of this lubricant remains on the surface of the tires and can cause a reduction in traction until it is completely worn away. Usually, about 500 miles of easy driving (accelerating, braking, cornering) will allow the lubricant to wear off.
Also, tires are manufactured with several layers of materials including rubber, steel, and fabric. Giving the tire some time to break-in will allow these different components to work together in order to deliver the maximum performance and ride quality for which they were designed.
Finally, if you have just replaced your tires, it is probably because your old tires had very little tread depth remaining. Tires with very little tread depth will tend to respond a bit quicker because there is less tread to flex during cornering and rapid turning. Tires with a deeper tread will tend to flex or “squirm” a bit more, which will feel slightly less responsive than your older tires, even if you have replaced your old tires with the exact same brand and model of tire.
In summary, be prepared to go easy on your new tires for a few hundred miles so that they can be properly broken-in.
Examples of mismatched tires include mixing winter and all-season or summer tires, tires with different tread patterns, tires from different manufacturers, run-flat tires with non-run-flat tires, and tires with different construction characteristics or sizes. The only exception to this would be if your vehicle has a staggered fitment with different sized tires on the front and rear axles of the vehicle.
In addition to maintaining optimum control and stability, having a uniform set of tires allows you to regularly rotate them. This will enable you to have even tire wear, thus extending their life and maximizing your investment in your tires.
Although it is ideal to have your tires wear evenly throughout their lives, it is not always possible, even with regular rotations. Tires can wear unevenly if your vehicle has staggered fitments on the front and back, if your vehicle has an alignment or other mechanical problem, if your vehicle is insufficiently maintained, or due to driving conditions. You may also lose an individual tire due to puncture or other irreparable damage. In either case, you may be forced to consider purchasing one or more tires to replace the worn and/or damaged tire(s), or having to buy a complete set and forfeiting the remaining life on the non-damaged tires.
Obviously, it is best to replace all the tires at the same time because it will restore the handling and traction to optimum levels. However, due to financial reasons, this is not always possible. On the other hand, the less-expensive alternative of replacing two of the tires requires choosing exact, equivalent, or alternative tires.
If you decide to replace just two of the tires, the best choice is to replace them with the exact same brand, style, and size as the tires already on the vehicle. This is wise because the tire’s physical dimensions, internal construction, materials, and tread design will be nearly identical to the tires you are replacing. If it is not possible to purchase the exact same tires as those currently needing to be replaced (for example, the tire is no longer available), then the next best option is to choose tires in the same performance category with equivalent load rating, speed rating, handling, and traction characteristics as the original tires. The final option of choosing alternative tires that do not match any of the characteristics of your original tires should only be used as a last resort or in an emergency situation. This is because the use of tires with different performance characteristics, load ratings, speed ratings, or sizes can cause the vehicle’s handling to become unbalanced, especially in adverse weather conditions.
In summary, always try to maintain an even set of tires that are identical in brand, model or style, size, tread pattern, and tread depth. Anything else will compromise your vehicle’s comfort, handling, and safety characteristics.
For more information on how and when to properly rotate your tires for even wearing, see the articles entitled Rotating Your Tires and Tire Rotation Patterns.
Clean Your Tires
Before you store tires, you should first clean them thoroughly. The tires may have accumulated brake dust, dirt, or road grime during driving. If these compounds are allowed to sit on the tires for a long period of time, they can damage the tires. Use soap, water, and a tire brush to clean the tires. If your tires are still mounted on wheels, you may also want to clean the wheels using an approved wheel cleaner. Use a towel to wipe the tires and wheels dry, and then let any remaining moisture evaporate, allowing the tires and wheels to dry completely. Do NOT use any additional tire dressings or shine products.
Store In Airtight Bags
Once your tires are clean and dry, put them into large, dark/black airtight plastic bags, such as lawn and garden bags. Try to remove as much air from the bags as possible before sealing them with tape. Some people use a vacuum cleaner with the hose attachment to draw out as much of the air from the storage bags as possible before sealing them. Keeping the air out of the bags will help keep the oil within the tire compounds from evaporating.
Seasonal Tire Totes
Some people have seasonal tire totes that they use to store and carry tires. While these totes are convenient and make carrying easier, they are not airtight and they do not necessarily protect the tires from the atmosphere. If you want to use seasonal totes, it is recommended that you first place the tires in plastic bags (as described above) prior to placing them inside the totes.
Keep Tires Out of Direct Sunlight
Whenever possible, keep the tires out of direct sunlight. Because tires are black, they absorb heat more than a reflective surface would. Just sitting in the sun, tires can easily heat up to over 120Â°F. Besides the heat, the sun’s ultraviolet rays can cause rubber to deteriorate more quickly than normal.
Keep Away From Ozone
Ozone also damages tires. Therefore, do not store the tires near machinery that can generate ozone such as motors, pumps, or furnaces.
Don’t Leave on a Stored Vehicle For Long Periods
If you have to store a vehicle for long periods of time, the constant weight on the tires can cause tire damage. Long-term inactivity is more harmful to tires than regular use because during regular use, the regular flexing of the tires keeps the oil and rubber compounds within the tire lubricated and supple.
Store In A Cool, Dry Location
When it comes time to store your tires, make sure you store them in a cool, dry location such as a basement, climate-controlled garage, or workshop. Avoid storing the tires outdoors, in a standard garage, or in an attic that can become very hot or cold and/or be exposed to extremely wet or humid seasonal conditions.
Storing Uncovered White-Stripe / White-Letter Tires
If you have tires with white letters or a white stripe and you choose not to enclose your tires in plastic bags before storing them, you should be careful about how you stack them. The compounds that make up the white portions of the tire are different than the compounds in the black portions- the tires actually have a layer of non-staining black rubber covering the side of the tire where the white letters or stripes are located. This is done to prevent the black of the tire from migrating onto and staining the white portions. Thus, make sure you stack the tires white-to-white and black-to-black so that the white sides are touching each other and the black sides are touching each other. This will help keep the white portions from being stained during storage.
All tires will experience some aging, no matter what precautions you take. However, following the above guidelines will help to prolong the life of your tires, should you need to store them for long periods of time.