Impossible to measure something like that with a tape measure. Well, maybe to ±1". Maybe get a huge set of calipers? Or just measure around it... that's the easiest way. Or just look up the tire size on any number of charts. The tire companies know the diameter of their tires when mounted on the rim they recommend, at standard inflation. Which I'm guessing most of us adhere to? If not, you'll need to start measuring, somehow, more accurately.
