Your tires are the only part of your car, truck, or SUV that actually touches the road. If the tires are ill-fitting, over-filled, or under-filled, this poses a risk to you and others. Using a tire ...

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.