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.

For example, a touring bike that's ridden long distances with lots of weight and isn't leaned over a great deal has a drastically different-size tire than a sportbike.