Lay the tire on the ground. Put a straight edge across the sidewall. Measure from the ground. Now you have the sidewall width of the tire. Then measure down from the sidewall to the outer lip of the wheel using a straight edge. Take this measurement and multiply by 2. Then add 1". Then take that answer and subtract it from the tire sidewall number. Example: Tire sidewall measurement is 12". Sidewall to outer rim lip measurement is 1.5".
1.5 x 2 = 3 3 + 1 + 4 12 - 4 = 8 The edges of the rim vary som with different wheels. Measurements may be off by a 1/4" or so. But it'll be close enough to tell
Doug