Bias-ply tires tend to be rounder than radial tires. Bias-ply tires date back to 1898. With the development of vulcanized rubber, Goodyear began creating bias-ply tires with cords of fabric built into the rubber for stability. The sizing on bias-ply tires may seem a bit odd, as it is largely based upon wheel-diameter. Most newer passenger vehicles use a P-metric system (such as P235/75R15) which is based on aspect ratios, side-wall heights, and rim diameters. Converting bias-ply to P-metric tire sizing is not difficult.
- Identify the bias-ply tire’s diameter. The bias-ply measurement is printed on the original tire or in the original specifications for the vehicle. Locate this measurement on the tire size chart included in the Resources section, and the diameter that is displayed next to the measurement size. For example, the A78-14 tire has a 24.2-inch diameter. If you are unable to locate the written measurement of your bias-ply tire, measure the tire from top to bottom to get the diameter.