How Do I Convert Old Tire Sizes?

While today's P-metric passenger tire sizes have existed since the early 1980s, restoring classic muscle cars and pony cars has kept yesterday's numeric and alpha numeric tires from disappearing. This chart has been developed to help you determine their equivalent Euro-metric and 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 an 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 are to be used. Today's lower profile sizes will usually result in too wide a tire with too much gap between the wheelwell opening and the top of the tire.

"The early numeric tires had the equivalent of a 90-series aspect ratio, while later tires offered a "lower" profile equivalent to an 80 series."

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 60s and became widely used in the early 70s. These tires were identified with a letter which indicated the tire's load capacity, followed by an "R" if radial ply construction, the tire's aspect ratio and wheel diameter.

So while G78-15, G70-15 and G60-15 sized tires are all rated to carry the same maximum load, their different aspect ratios resulted in tires with the overall diameters indicated below.

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 replaced with a P235/60R15 (selected from the P-metric 60-series column).

This chart can help you determine today's closest equivalent Euro-metric or P-metric tire sizes for popular 1949 through 1980 tire sizes.

• Listing in chart does not imply complete interchangeability.
• When changing tire sizes, dimensional clearances must be checked.