Most cars, SUVs, and light trucks (½-ton or smaller) have P-metric or Euro-Metric tire sizing scales. If your vehicle’s tires list the letter “P” before the tire number sequence, your tire size is standard for passenger car tires. If there is no letter preceding the tire number sequence, your tire size will be measured in Euro-Metric sizing.
Some heavy-duty SUVs, trucks, or vans that are typically ¾-ton or larger will be measured using either LT Metric systems or Euro-Metric Commercial systems. If your tire is LT-Metric, the letters “LT” will be listed before the number sequence begins. Tire’s measured in Euro-Metric Commercial, or C Type, measurements will have a letter “C” listed directly after the rim diameter number in the sequence.
If your vehicle does not have a full-size spare tire, it may have a compact spare, or donut, located in the trunk or under the vehicle. Your vehicle’s temporary or compact spare tire size will feature a letter “T” listed before the tire number sequence. Trailer tires designed for on-road hauling but not for use on passenger vehicles will be labeled with an “ST” before the tire size sequence.
If it’s time to replace your vehicle’s tires, or if you’re inspecting tires on a used vehicle, knowing how to read the important information located on the side of your tire is necessary to understand a variety of specifics about the tire. This information can help you purchase the correct replacement tires, ensure that the tires on your vehicle are appropriate for the way you drive, and determine the tire's age, speed and weight ratings, and more. All tires list important and specific information regarding the tire’s size, maximum inflation pressure, tire DOT numbers, and other codes. This information can be found on the outer sidewall of your tires, but may just seem like a confusing list of numbers and symbols if you’re not sure how to decipher their meaning. Read on to learn what each piece of data on your tire represents and how to utilize the information on your tires.
There is a variety of important pieces of information listed on the sidewall of your tires. Knowing what each symbol and number means allows you to properly read and determine your tire’s specifications, and help you to more easily replace, repair, and maintain your tires.
Tire Size
Every tire manufactured includes its tire size, which is listed on the sidewall of the tire. Tire size formatting includes elements like width, aspect ratio, construction, rim diameter, loaf index, and speed rating. For more information on reading tire size, read on below.
Department of Transportation Code
The Department of Transportation Code, abbreviated DOT on your tire, is a stamp of approval from the Department of Transportation indicating your tire has passed all required safety standard tests. The TIN, or tire identification number, represents the tire manufacturer identification, and will be the next group of numbers and letters following the DOT mark. In some cases, one side of the tire will list a partial TIN and the other will list a full TIN. The full TIN consists of four symbol groupings separated by spaces. The first number grouping represents the tire’s manufacturing plant code and ID. The second grouping lists the tire’s size code. The third grouping lists the manufacturer construction code. Lastly, the fourth grouping lists the date the tire was manufactured, listed in the order by the week number and followed by the year.
Uniform Tire Quality Grading Code
The Uniform Tire Quality Grading Code, printed as UTQG on your tire, is the result of a series of government-regulated assessments created by the National Highway Traffic Safety Administration (NHTSA) to test a tire’s treadwear, traction, and temperature. These ratings are all listed on the sidewall of your tire.
Treadwear provides a prediction of approximately how long the tread on your tires should last based on a series of tests on a government course. Treadware is not based on mileage, but a numeric percentage of how long your tires tread is predicted to last in comparison to a standard reference tire used as the control in the government assessment. If your tire has a treadwear rating of 200, and the reference tire is standardized at 100, your tread is predicted to last twice as long as the reference tire tread. However, if your tread is 50, your tread is expected to only last half as long as the reference tire’s tread.
Traction grade specifies the amount of grip or traction and control your tire is expected to have on wet surfaces. Grades include the following: AA, A, B, and C, with AA being the highest traction grade. AA-graded tires are typically high-performance tires and are capable of maintaining the best possible traction at any speed and temperature, and are capable of stopping in much shorter distances than lower-traction tires. Any traction grade below a C is deemed unsafe and unacceptable for on-road travel, but may be suitable for off-road, agricultural, or other uses.
Temperature grade is based on your tire’s level of heat resistance. Temperature grades range from A to C, with A being the most heat-resistant grade. If you have racing, all-season, or all-weather tires, you will most likely have a temperature grade of A, whereas standard street tires may be rated at a grade of B or C.
Winter Tires
Some tires are designed to deliver high-quality performance in specific environments or seasons, and winter tires are the most common seasonal tire produced. These tires are often designed for optimal performance in snow and icy conditions, and are often a necessity if you live or drive in harsh winter climates. The increased traction grade on these tires will provide superior braking and stopping performance, increased traction, and better turning or cornering in icy, snowy, or wet road conditions. Winter or snow tires will often be indicated by a snowflake and mountain symbol on the sidewall of your tire. In some cases, winter or snow tires may even be designed with specific tread to wick snow away from the center of the tire or include studs in the tread for added traction. Be sure to consult your state and local officials regarding the use of winter tires to be aware of any winter tire laws or regulations in your area.
Width
The tire width is the first measurement in the number sequence. Tire width refers to the measurement in millimeters from one sidewall to the other at the outermost point of the tire.
Aspect Ratio
The tire’s aspect ratio is listed after the width measurement, directly after the dash or slash. Aspect ratio measures the tires profile to help you determine how tall it is. To find the aspect ratio, the tire's sidewall height from the rim to the tread is divided by the tire’s width. This aspect ratio on your tire simply represents a percentage that compares the sidewall height to the width of the tire. Lower aspect ratios typically deliver higher-quality performance when braking and handling in both wet and dry road conditions by limiting the amount of flex in the sidewall, and even tires that have the same sidewall height but different widths will have a different aspect ratio.
Construction
Construction, when referring to tires, relates to the internal makeup and design of a tire. Radial tires are the most common type of tire on the road today, and are known for their increased stability and control while driving. If your tire has the letter “R” listed in the tire design sequence, this means it is engineered with radial construction. Radial construction is made up of made up of a supportive layer of steel plies or threads that run at 90-degree angles throughout the tire. Bias-ply and diagonal tires are two other types of tire constructions, and will be represented with the letters “B” or “D”. Bias-ply tire threads are nylon-based, and are often sensitive to overheating. Diagonal tires are also threaded with nylon cords that run diagonally through the tire, overlapping one another.
Rim Diameter
Rim diameter is the next number in the sequence, and is used to provide the diameter measurement of the wheel in inches. If the rim diameter is listed as 16 in the sequence, the tire is manufactured specifically to fit a rim with a 16 inch diameter. If you choose to change the size of your tires and want to continue to use the same wheels, this is the only measurement that can’t be changed in many cases.
Load Index
The next number in the sequence represents the tire’s load index. This number specifies how much weight in pounds the tire is able to carry and support when inflated to the correct pressure. These numbers are represented across a scale, and each value corresponds to its match on the load index carrying capacity limit. To find the specific carrying capacity for your tires, refer to your owners manual or tire manufacturer for the correct guide specific to your tires. There are two load types for passenger vehicle tires: standard load, and extra load. If your tires are specified as extra load tires, it will be represented with an “KL” after the size and load index in the tire’s number sequence.
Speed Rating
The final figure in your tire sequence is the speed rating, which acts as a performance rating for driving your vehicle. This figure represents the top speed at which your vehicle’s can perform properly and safely. The higher the speed rating, the better overall performance and control your tires will likely provide. The letters and codes of the speed rating are alphabetized and can be anywhere from A to Z, A being the lowest and Z being the highest rating.It is important to remember that while your tire can have a speed rating of up to 186 miles per hour, you should not exceed the legal speed limit for the area in which you drive. To ensure optimum safety while driving your vehicle, each tire should have the same speed rating. If speed levels differ for any reason, always drive in accordance with the tire with the lowest speed rating.
Flotation sizing is often used in off-road truck or SUV tires. While floatation tires and metric tires look similar, flotation tires are designed for off-road conditions that may include sinking soil or unstable ground compositions. While flotation tire sizes do have equivalents in metric tire size, in some cases, their load capacities may differ slightly.
Overall Diameter
The first number in a flotation tire measurement sequence represents the diameter of the tire in inches.
Section Width
The second number in the flotation tire measurement sequence is the width of the tire in inches. This measurement will be from sidewall to sidewall on the tire at the outer edge.
Internal Construction
The internal construction of the flotation tire is the next number in the sequence. The types of tire construction are the same as those used in Metric measuring, and can be either radial (R), which is the most common tire construction, diagonal (D), or bias-ply (B).
Rim Diameter
The fourth number in the sequence will value the total diameter of the wheel in inches.
LT Type
Light Truck-type vehicle tires will be labeled with the letters “LT” after the rim diameter in the number sequence.
Load Index and Speed Rating
The last two symbols in the tire size sequence represent the load index and speed rating. Reading these two measurements for flotation tires is the same process as reading them for metric tires.
Ensuring you know the correct size, pressure, date, speed, and load codes for your tires is the best way to prevent any unnecessary needed repairs that can arise if you use tires not suitable for your vehicle. Being able to read and decode the information on your tires can help you stay prepared and better understand the recommendations you should follow to maintain your tire’s best overall performance on the road. In addition to your tire’s sidewall, the manufacturer-recommended inflation pressure and tire size can also often be found on the driver’s side door frame. This label can provide you with the details you need to know to properly maintain and replace your tires as needed. You can also find this information in your owners manual, along with other helpful tire maintenance tips, including how to check tire pressure, when to replace your tires, how to rotate your tires, and other tire pressure tips and information.
How To Read Tire Size
Knowing how to read the tire size on the sidewall of your tires is a great skill for all car owners to have. The basic elements of tire size are width, aspect ratio, and rim size. The first number is the tires width in millimeters, then the aspect ratio – a percentage of the tires sidewall measurement divided by its width – followed by the rim size in inches.
Why Are Tire Codes Important?
DOT codes, UTQG codes, and other detailed information on a tire’s sidewall are all essential for safety, maintenance, and knowledge regarding needed repairs. These codes are designed to set safety standards for all types of tires on the road. Knowing how to read these codes and what each code stands for prepares you for any tire maintenance or repair needs and gives you details that can inform you about specific functions and recommendations for your tires.
When Should Tires Be Replaced?
Typically, tire manufacturers urge vehicle owners to replace tires after they have been used anywhere from six to ten years, depending on the tire’s tread wear resistance. However, tires may need to be replaced sooner if any physical damage occurs, such as puncture wounds, flat tires, shredded tread, dry rot or cracking, or air leaks. For more information and recommendations on how to check tire tread and when to replace your tires, check out our helpful article.
What Are Directional Tires?
Directional tires are manufactured with a tread design that is made to rotate in only one direction. Directional tires are marked with a small arrow, pointing in the direction that the tires should be rotating as the roll forward. If you own or purchase directional tires, pay close attention to these arrows as you put the tires on your vehicle or take the tires off. When the tire is mounted correctly, the arrow will be facing the front end of the vehicle.
Where Can I Find More Information About Tires?
Information about your tires can be found in a number of places, and is generally easy to locate once you know what you’re looking for. Aside from the sidewall of your tire, if your vehicle is equipped with the same size and type of tires as it had from the factory, you can find DOT codes and sizing information on your driver side door frame, in your vehicles owners manual, or by contacting your tire’s manufacturer. For more information about tire sizing and requirements, visit National Highway Traffic Safety Administration (NHTSA).