Lay down the wheel and tire on a flat 1 x 4 on your work bench. Lay another 1 x 4 on top of the tire.
Measure in between the wood at the outside of the tire. Then measure between the wood and the rim at the bead area.
Let's say the first measurement is 7 inches and there is 1 inch at the second measurement on both sides. Your rims would be 5 inches outside and would most likely be 4.5 inches inside width.
