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Choosing Tires: Cracking the Sidewall Code

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Author: Guy Goodyear

There is a lot of information to be found on your tire's sidewall. However, aside from the part that gives you the maximum load and the maximum tire pressure, it's not exactly easy to understand.

There is a lot of information to be found on your tire’s sidewall. However, aside from the part that gives you the maximum load and the maximum tire pressure, it’s not exactly easy to understand.

A good place to start is by taking a look at the Uniform Tire Quality Grading (UTQG). It’s one of the few places on the tire where you will see actual words. These are tread wear, traction, and temperature.

Tread wear is followed by a number which tells you how quickly the tire will wear. These numbers are a comparative scale developed by testing the tires under controlled conditions. A tire with a tread wear rating of 150 will last 1.5 times as long with a tire with a rating of 100.

There’s no such thing as a free lunch though. As the tread wear rating rises, traction usually decreases. There are four ratings for traction: AA, A, B, C. AA is the best. Temperature rating follows a similar system, with codes ranging from A to C with A being the best.

If you want to know how well the tire handles under adverse conditions, there are two things to check for. One is the mud and snow designation, which may be abbreviated M+S, M&S or M/S. this designation appears on both snow tires and all-season radials.

Since 2001, a new designation has been added to tires designed with severe winter driving in mind. This designation is represented by a stylized mountain and snowflake, and it is a great help in distinguishing all–season radials from true snow tires. If you live in an area that sees a lot of snow you may want to invest in tires that have this designation.

Now we come to the part of the code that is the hardest to crack, the tire size, load index, and speed symbol. Passenger cars, light trucks, and heavy trucks all use different sizing systems, as tires from one type are not normally used on another. There are some exceptions to this, as some light trucks and SUVs use tires intended for cars.

Complicating matters slightly is the fact that there are three commonly used systems for passenger cars alone. The first is known as P-Metric. A code in this system will read something like P225/65R15 96H. This is virtually identical to the system used by light trucks. The only difference is that instead of a P denoting passenger car it reads LT for light truck.

The number after the P (225 in our example) is the section width in millimeters measured at the sidewall’s widest point when the tire is fully inflated. The number after the slash (65) shows the aspect ratio, which shows the relationship of the section height to the section width. The lower this number is, the wider the tire compared to the height of the sidewall.

The next item (R) denotes radial construction. The number following this (15) is the rim diameter in inches. The last part of the code (96H) is the service description. The numerical part is the load index and the letter code is the speed rating. The load index is an assigned number that shows the maximum load for that tire. The speed rating is based on rigorous scientific testing that measures tire durability under high speed conditions. Please bear in mind that these tests do not take into account tires that may be over inflated, under inflated, or damaged.

Shopping tires can be a painful business, but if you take the time to learn what the tire is telling you the whole process will go a lot smoother.

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