The tires’ size will be prominently listed on the sidewall. This is a string of letters and numbers that usually starts with “P.” For this example, we’ll use P215/55R17 tires, found on a ...

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.

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.