running at high speed. Grade C is the minimum level of performance for all passenger car tires as set under
Federal Motor Vehicle Safety Standards. This grade is established for a tire that is properly inflated and not
Note: UTQG ratings are not required on winter and light truck sized tires.
Calculating Tire Dimensions
Modern metric tire dimensions Example...185/60R14 85H or 185/60HR14
The first number is the width of the tire in millimeters, measured from sidewall to sidewall. To convert to
inches, divide by 25.4 In the example above, the width is 185mm or 7.28".
The second number is the aspect ratio. This is a ratio of sidewall height to width. In the example above, the
tire is 7.28" wide, multiply that by the aspect ratio to find the height of one sidewall. In this case,
185x0.60=111mm or 7.28"x0.60=4.36".
The last number is the diameter of the wheel in inches.
To figure the outside diameter of a tire, take the sidewall height and multiply by 2,(remember that the
diameter is made up of 2 sidewalls, the one above the wheel, and the one below the wheel) and add the
diameter of the wheel to get your answer.
Example...185/60R14 85H or 185/60HR14
185mm x .60=111mm x 2=222mm + 355.6mm(14")= 577.6mm or 22.74"
Tire Size Conversion Chart
While today's P-metric passenger tire sizes have existed since the early 1980's, restoring classic muscle cars
and ponycars has kept yesterday's Numeric and Alpha Numeric tires from disappearing. The chart below is
intended to help you determine their equivalent P-metric tire sizes.