I think the tire diameters went in increments. So the smallest was 10", then 12", then jumped to 16" then to 20". I think a good rough measurement to chose a tire size is to measure the remaining rim diameter and then add 3" and judge if that process yields a 10', 12" 16" or 20" size. Of course using a wire on tire is easier. I can install a wire on if you send me the wheel.