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How to Read Tire Numbers

Tire sizes for both passenger vehicles and light trucks are usually represented by a combination of numbers and letters printed on the sidewall – including a mandatory U.S. Department of Transportation (D.O.T.) approval code.


Below are tips and information to help you learn how to read tire size and other tire markings commonly seen in the U.S.

Original Equipment Approval Code
All tires must have a DOT, Department of Transportation number which indicates the tire has passed all minimum DOT standards for sale in the U.S.

Uniform Tire Quality Grading (UTQG) Marking
The Department of Transportation requires each manufacturer to grade its tires under the Uniform Tire Quality Grade (UTQG) system which establishes ratings for treadwear, traction and temperature resistance. These tests are conducted independently by each manufacturer following government guidelines that provide a comparison between the manufacturers’ tested tire and a control tire.

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Tire Size Markings
The tire size shown above is 225/50R16. The 225 represents its section width. The number "50" indicates the tire’s aspect ratio. The last number, "16" indicates wheel diameter.

  • Section Width
    Section Width is the width of the tire from one sidewall to the other sidewall in millimeters. The tire numbers in the above example tell us that the tire is 225 millimeters wide.
  • Aspect Ratio
    Aspect Ratio is the ratio of the sidewall height to the section width. The sidewall height of the 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 1,389 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.