The 10.50 inches is the tire's advertised width, usually taken at the tread, not the sidewall bulge. And the 15 is the size of the wheel it's designed for.

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 ...