This is probably the moron approach (my way), and is harder to explain than do...
Picture 2 1x2s clamped by rubber bands on opposite sides of the tire.
Measure the width of the tire at its widest point, or the distance between the 2 1x2s.
Measure from the 1x2 to the outer bead seat. Do the same for the inner.
Subtract those measurements from the tire width and subtract 1/4" for the thicknesses of the wheel.
Round to the nearest half inch.