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.

We’ll tell you how to crack the code so you can understand what tire sizes mean. ... You can also take a look at the tire size chart below, ... The diameter of the wheel (or rim) in inches. 92.