And the 15 is the size of the wheel it's designed for. So, in this case you've got a 31-inch tall tire for a 15-inch wheel. You know the wheel radius is 7.5-inches, and the tire radius is 15.5 inches.

3. Mark where the paper overlaps with a pen and measure the length in millimeters. 4. Click here to download and print our guide to scale. Use a calculator to divide the length you measured by 3.14.