The diameter of the wheel is actually measured from the bead surface, not the outer lip. If you were to take a 15" tire and measure the distance from bead to bead stright across, it would be 15", but if you measure your rim from the outside of the lip stright across to the outside of the other lip, it will be 16". This is because the lip on the outside of the wheel is 1/2" taller than the bead surface. A 16" wheel will actually measure 17"!