The size of most tires for both passenger vehicles and light trucks are represented by a combination of numbers and letters that are printed on the sidewall of your tires.
Below are the tire markings most commonly seen on tires sold in the US. All tires must have a US Department of Transportation (D.O.T.) approval.
Original Equipment Approval Code
All tires must have a DOT, Department of Transportation numbers which indicate that the tire has passed all minimum DOT standards.
Uniform Tire Quality Grading (UTQG) Marking
The Department of Transportation requires each manufacturer to grade its tires under the Uniform Tire Quality Grade (UTQG) labeling system and establish ratings for treadwear, traction and temperature resistance. These tests are conducted independently by each manufacturer following government guidelines to assign values that represent a comparison between the tested tire and a control tire. While traction and temperature resistance ratings are specific performance levels, the treadwear ratings are assigned by manufacturers following field testing and are most accurate when comparing tires of the same brand.
Tire Size Markings
The tire size shown above is 225/50R16. The 225 represents its section width. "50" is the tire’s aspect ratio, and the last item is the "16" which represents the wheel diameter.
• Section Width
Section Width is the width of the tire from one sidewall to the other sidewall in millimeters. This tire in the example above is 225 millimeters wide.
• Aspect Ratio
Aspect Ratio is the ratio of the sidewall height to the section width. The sidewall height of example tire above is 50% of its section width.
• Wheel Diameter
Wheel Diameter specifies the size, in inches, of the wheel that a tire fits. The example tire will only fit a 16-inch wheel.
Other Tire Markings • Construction
The "R" refers to the example tire’s “radial” construction. Radial tires have layers of fabric whose cords run at right angles to the circumference of the tire and whose tread is strengthened by additional layers around the circumference.
• Load Index
Load Index indicates the tires Load Carrying Capacity. The example tire’s Load Index is 92, which means it can carry loads up to 1389 pounds.
• Speed Rating
Speed Rating designates the maximum speed at which a properly installed and inflated tire may be driven. The example tire’s Speed Rating is V, which means it can be driven at speeds up to 149 mph.