Each section of small print on a tire's sidewall means something:
The P designates that the tire is a passenger vehicle tire. Some other designations are LT for light truck, and T for temporary, or spare tires.
The 235 is the width of the tire in millimeters (mm), measured from sidewall to sidewall. Since this measure is affected by the width of the rim, the measurement is for the tire when it is on its intended rim size.
This number tells you the height of the tire, from the bead to the top of the tread. This is described as a percentage of the tire width. In our example, the aspect ratio is 75, so the tire's height is 75 percent of its width, or 176.25 mm ( .75 x 235 = 176.25 mm, or 6.94 in). The smaller the aspect ratio, the wider the tire in relation to its height.
High performance tires usually have a lower aspect ratio than other tires. This is because tires with a lower aspect ratio provide better lateral stability. When a car goes around a turn lateral forces are generated and the tire must resist these forces. Tires with a lower profile have shorter, stiffer sidewalls so they resist cornering forces better.
The R designates that the tire was made using radial construction. This is the most common type of tire construction. Older tires were made using diagonal bias (D) or bias belted (B) construction. A separate note indicates how many plies make up the sidewall of the tire and the tread.
This number specifies, in inches, the wheel rim diameter the tire is designed for.
Uniform Tire Quality Grading
Passenger car tires also have a grade on them as part of the uniform tire quality grading (UTQG) system. You can check the UTQG rating for your tires on this page maintained by the U.S. National Highway Traffic Safety Administration (NHTSA). Your tire's UTQG rating tells you three things:
- Tread Wear: This number comes from testing the tire in controlled conditions on a government test track. The higher the number, the longer you can expect the tread to last. Since no one will drive his or her car on exactly the same surfaces and at the same speeds as the government test track, the number is not an accurate indicator of how long your tread will actually last. It's a good relative measure, however: You can expect a tire with a larger number to last longer than one with a smaller number.
- Traction: Tire traction is rated AA, A, B or C, with AA at the top of the scale. This rating is based on the tire's ability to stop a car on wet concrete and asphalt. It does not indicate the tire's cornering ability. According to this NHTSA page, the Firestone Wilderness AT and Radial ATX II tires that have been in the news have a traction rating of B.
- Temperature: The tire temperature ratings are A, B or C. The rating is a measure of how well the tire dissipates heat and how well it handles the buildup of heat. The temperature grade applies to a properly inflated tire that is not overloaded. Underinflation, overloading or excessive speed can lead to more heat buildup. Excessive heat buildup can cause tires to wear out faster, or could even lead to tire failure. According to this NHTSA page, the Firestone Wilderness AT and Radial ATX II tires have a temperature rating of C.
The service description consists of two things:
- Load Ratings: The load rating is a number that correlates to the maximum rated load for that tire. A higher number indicates that the tire has a higher load capacity. The rating "105," for example, corresponds to a load capacity of 2039 pounds (924.87 kg). A separate note on the tire indicates the load rating at a given inflation pressure.
- Speed Rating: The letter that follows the load rating indicates the maximum speed allowable for this tire (as long as the weight is at or below the rated load). For instance, S indicates that the tire can handle speeds up to 112 mph (180.246 kph). See the chart on this page for all the ratings.
Calculating the Tire Diameter
Now that we know what these numbers mean, we can calculate the overall diameter of a tire. We multiply the tire width by the aspect ratio to get the height of the tire.
Then we add twice the tire height to the rim diameter.
This is the unloaded diameter; as soon as any weight is put on the tire, the diameter will decrease.