Everything you need to know about a tire is on the sidewall, except, of course, the price. There are letters and numbers molded into the rubber, each telling valuable details about the characteristics of that tire. At first glance, all of these characters can be intimidating. But, with a little explanation, it’s easy to decipher the meaning of each.
If you’re in the market for tires, the size code will be the first consideration for selecting the right tire. Check your owner’s manual or driver’s side door jamb to verify the manufacturer recommended size for your vehicle.
Type The letter P (for passenger car) at the beginning of the tire size code means that this tire is made for light-duty service, usually a car or smaller pickup truck. You may also see the letters LT (for light truck) before the tire size, which would mean the tire is designed for a bit heavier service on a pickup truck or SUV.
Width Next in the tire size is 205, which is the section width measurement in millimeters from sidewall to sidewall. The tire pictured is 205 mm wide.
Aspect ratio The number after the slash is the aspect ratio. This refers to the profile height of the sidewall, or the height of the sidewall as a percentage of the tread width. In our example, the 55 means that the sidewall height is 55% of the 205-mm tread width.
Construction The letter following the profile height identifies the tire’s construction. Most tires sold today are an R, which means they are of radial tire construction. Though not nearly as common, other construction codes you might see are D for diagonal and B for bias belted.
Wheel diameter The final number in the size code is the diameter — in inches — of the wheel on which the tire will be mounted. This tire would be mounted on a 19" wheel.
Load index and speed rating The last grouping of characters, tied to the tire type, size and construction, represents the load index and speed rating. Also known as the service description, the number indicates the maximum load the tire can support when inflated properly, and the letter represents the maximum speed capability of the tire. In our example, a load index of 92 means that the tire can support 1,389 pounds at maximum air pressure. Multiply that by four tires (4 x 1,389 = 5,556 pounds) to get your vehicle’s maximum load–carrying capacity. The letter T speed rating indicates maximum speed capability of 118 mph.
Although the size, construction and type will be the minimum information to arm yourself with when buying tires, a variety of other important details can be found in the other letters, numbers and symbols on the tire.
U.S. Department of Transportation Code
All tires must have a U.S. Department of Transportation (DOT) number, which indicates that the tire has passed all minimum DOT standards for sale in the United States. This code represents the manufacturer and the factory where the tire was made, as well as manufacturer-specific coding used for tracking in the case of a recall. The last four numbers give the date of production — the first two indicate which of the 52 weeks in a year it was made, and the second two numbers represent the year. For example, 5200 indicates that a tire was manufactured during the 52nd week of the year 2000.
Uniform Tire Quality Grade Standards
The Department of Transportation also requires each manufacturer to grade its tires under the Uniform Tire Quality Grade (UTQG) standards, which establishes ratings for treadwear, traction and temperature resistance. These values are determined by the manufacturer.
Treadwear The treadwear grading is a relative figure based on the rate of wear a tire displays during a 7,200-mile on-vehicle test. The higher the number, the longer the tire will likely last. For example, a treadwear value of 600 should deliver more mileage than one of 400.
Traction The traction rating refers to the tire’s ability to stop in a straight line on wet asphalt and concrete under the controlled conditions of a test track. The possible traction grades are AA, A, B or C, with AA being the best stopping ability and best traction.
Temperature The temperature rating offers information about the tire’s resistance to heat when driven at high speeds. The grades range from A to C. Just like in school, you will want an A, as these tires are the most effective at dissipating heat.
M+S, or M&S
These letters stand formud and snow, and you’ll see them only on all-weather tires designed for traction in muddy conditions or light snow.
The mountain with a snowflake symbol means that the tire is classified as a winter tire. Studded winter tires will have an additional letter, E (for extreme) (M+SE).
Some tread designs are “directional” or “unidirectional,” which means they are designed to be installed with the tire facing a specific direction. These tires will have an arrow showing which way the tire should rotate when the vehicle is moving forward.
Some tire tread patterns are asymmetric, which means that the tread pattern is different when comparing the right side of the tread to the left. These patterns are designed to improve handling and must be mounted with the proper pattern to the outside of the vehicle. On these tires, you will see “Mount Outside” or similar cautions on the sidewall to ensure that the correct side of the tire is facing outward.
As you can see, a tire’s sidewall tells you a lot. Whether you’re in the market for new tires or just looking to understand what you currently have, this should be more than enough to get you started.
One thing to remember about ratings: Every manufacturer follows their own set of standards, even those required by the DOT. Keep that in mind when comparing tires and know that the rating will be most accurate when comparing tires made by the same manufacturer.