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.

Even though the 5-inch size difference doesn’t seem like much, a 37-inch tire reduces the 3.73 gear ratio down to about a 3.19. See All 71 Photos ...