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What mean inscriptions on your tires?

Car maintenance is an essential part of car ownership, and one element that gets a lot of car owners confused is tire reading. If you take a look at your car’s tires, you will notice a lot of numbers on the sidewall. Knowing how to read these numbers and understand their meanings can give you a valuable insight into your tires and help you in ensuring their proper upkeep.

Tire code
Tire identification diagram

 

The Numbers On The Outer Edge

The first numbers you might notice on your tires are the set of numbers at the outer edge. The first of these numbers denotes the width of the tire from one sidewall to the other in millimeters. If the number has a “P” at the front of it, it means it is a US tire (also known as “P-Metric”), and if not then it is a European metric tire. The two have slight differences between them in terms of the way the load ratings are calculated, but these differences are very slight.

The next number is the aspect ratio, which shows the height of the tire from the rim to the tire’s top as a percentage of the tire’s width. If, for example, the ratio is denoted by the number “65”, this means that the height of the tire is 65% of the tire’s total width.

After the aspect ratio, the inside diameter of the tire is noted, measured in inches. This number doubles as the outside diameter of the tire’s rim. If this number is preceded by an “R”, it means that the tire’s plies run perpendicular to direction in which the tire spins, rather than overlapping (which is known as “bias-ply”).

tire-determining

Lastly, these outside numbers note the load index and speed rating of the tire. The load index corresponds to the allowed load that the tire can bear. The exact load ratings can be found online, but as an example tires with a load index of 96 can bear 1,565 lbs. Across four tires, this amounts to 6,260 lbs. The speed rating relates to the top speed the tire is designed to sustain over a prolonged period of time. The most common speed rating is V, which amounts to 149 miles per hour.

 

Other Important Numbers To Note

The tire’s identification number can also be found on the tire’s sidewall, and acts as an assurance that the tire has met Federal and Department of Transportation standards (denoted by the letters before the dot). After the dot, the numbers read thusly: the first two indicate where the tire was made and the next four when it was manufactured (e.g. 1510 shows that the tire was assembled in the 15th week of 2010.) These are important numbers to remember as they are what are used for when identifying recalls. The markings on the edge of the sidewall appear when the tire has worn to the point of legal baldness.

ST Tire DOT B
Tire’s identification number

The inner sidewall lists the tire ply composition, which lists the layers which comprise the tire as well as the materials used. A greater number of plies indicated higher load bearing.

The numbers and letters noted near the middle of the tire are the treadwear grade, the traction grade and the temperature grade. The treadware grade indicates how long the tire’s tread should last (based on 8,000 miles of testing), the traction grade rates the tire’s aptitude for stopping on wet roads (indicated by letters with AA being the best for wet roads, ranging down through A, B and C), and the temperature grade shows how well the tire resists heat (again graded as A, B and C). These numbers and letters make up the Uniform Tire Quality Grading (UTQG) standards.

UTQG
UTQG

Around the interior of tire you will also be able to see the max cold inflation limit, which indicates the absolute maximum air pressure the tire can take. However, it is recommended that you follow the proper inflation found insider the driver’s doorjamb. The number is measure in pounds per square inch.

Next to the tire’s name you will see the ECE (Economic Commission for Europe) approval mark, which indicates that the tire meets their standards. Other symbols which may appear on your tire are symbols indicating the tire’s weather design, such as “M+S” for mud and snow and a snowflake indicating that the tire is suitable for US and Canadian winter and has met those standards.

MS_SnowF

Understanding these numbers is an essential part of tire maintenance and can be immensely helpful in the proper upkeep of your tires.