Tire Conversion: Metric to Inches and Vice Versa

Tire Conversion

If you’re restoring a classic car or looking to turn your truck or SUV to an off-roading machine, you may find yourself needing to convert metric tire size measurements to inches to know the overall diameter. Our simple tire size conversion chart clearly lists the various metric tire size designations side-by-side with their size equivalents in inches.

Tire size is designated by the long sequence of letters and numbers branded on your tires’ sidewalls. This chain of characters includes information about your tires’ class or intended purpose, their dimensions, their load range (also known as ply rating) and their maximum speed capability.

For the purposes of converting metric measurements to inches for a tire’s overall diameter, look specifically at rim and wheel diameters and the conversions associated with those dimensions.

Rim or wheel diameter conversion

The equations needed to figure out the correct replacement of a metric tire with an inch-based tire can be a bit tedious. Especially when the metric system for tires can be difficult to decipher. The good news is that our tire size conversion chart will do all the work for you.

If you were to attempt to calculate the overall diameter of a metric tire in inches on your own, you would need to consider tire width, wheel diameter, and sidewall height or aspect ratio.

Example size: 215/60R15

Starting with the first three numbers, 215, convert your tire’s width from millimeters to inches by dividing by 25.4:

215mm / 25.4 = 8.46”

The next number, 60, represents your tire’s aspect ratio percentage. Calculate your section height by multiplying your width by your aspect ratio percentage:

215mm x 0.60 = 129mm

8.46” x 0.60 = 5.08”

The number 15 represents your wheel’s diameter in inches. This can be converted into millimeters by multiplying 15 by 25.4:

15” x 25.4 = 381mm

To calculate the tire diameter, multiply your section height by two and add the wheel diameter:

129mm + 129mm + 381mm = 639mm

5.08” + 5.08” + 15” = 25.16

Share this post