get the wheel size, x/y/z
- take tire width X and divide by 25.4 to convert from mm to inches (1)
- take (1) and multiply by aspect ratio Y as a percent (2)
- take (2) and multiply by 2 because there are 2 sections of sidewall as you calculate the rolling diameter (3)
- take (3) and add rim diameter Z to get the overall rolling diameter in inches

X/25.4=(1)
(1)*0.Y=(2)
(2)*2=(3)
(3)+Z=overall rolling diameter