George, find a couple of pieces of something that will span the rim of wheel. Have somebody hold one of those across the back of the wheel and stick a tape measure thru the wheel and touch or hook onto it. Place the other piece across the rim on the outside and see what reading you get from the tape measure. You can also do it by yourself, just need to get creative, like set the piece that is behind the wheel on a can or something at the right height.