SIZES, DIMENSIONS, & CONVERSION

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. Using the example above, the width is 185mm or 7.28".

The second number is the aspect ratio. This is a ratio in percentage 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 find the diameter of a tire you can:

A. 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"

B. Measure the circumference of the outside of the tire and divide by 3.14 (value of pi)

C. Measure from the center of the rim to the outside of the tire to get the radius and multiply by 2.

Tire Size Conversion   Tire Size Conversion Chart

While today's P-metric passenger tire sizes have existed since the early 1980's, restoring classic muscle cars 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.

Two versions of numeric tire sizes were used as original equipment on vehicles between 1949 and 1970. The early numeric tires had the equivalent of a 90-series aspect ratio, while later tires offered a "lower" profile equivalent to a 80-series. These tires typically featured tread widths ranging from 3.5" for the smallest 13" rim diameter tires to about 5.5" for the largest 15" rim diameter tires.

To convert Numeric sizes to today's P-metric sizes, it is important to remember that early cars were not only equipped with narrow tires, they were equipped with narrow wheels as well. In most cases, numeric tires should be replaced with today's 80- or 75-series sized tires. This is especially important if the original wheels or replicas are to be used. Today's lower profile sizes will usually result in too wide a tire with too much gap between the wheel well opening and the top of the tire.

For example: The 1965 Ford Mustang's 6.95-14 would be replaced with a P185/75R14.

Alpha numeric tire sizes were introduced as original equipment in the late 60's and became widely used in the early 70's. These tires were identified with a letter which indicated the tire's load capacity, followed by a "R" if radial ply construction, the tire's aspect ratio and wheel diameter. So while G78-15 (28.01"), G70-15 (27.5") and G60-15 (26.4") sized tires are all rated to carry the same load, their different aspect ratios resulted in tires with the overall diameters indicated above.

To convert Alpha Numeric tires to P-metric sized tires, it is important to identify the original tire's aspect ratio. The 78-series Alpha Numeric tires should be replaced with today's 80-, or 75 series tires. If the vehicle was equipped with the low profile 70-, 60- or 50-series sizes, the P-metric substitution should be selected from the P-metric size column that offers the equivalent aspect ratio as the existing tire.

For example: The 1970 Chevrolet Camaro Z-28 F60-15 would be replaced with a P235/60R15 (selected from the P-metric 60-series column).