When I was racing HO, I had a Tire Size Gauge that was simple to use. Basically, sized holes in a metal plate that you slid the tires into. When it wouldn't fit, you went up one size and that was your diameter.
I looked everywhere, but couldn't find anything similar for my 1/24th flexi cars. So, out came the CAD program, and with some laser cutting on a piece of 1/8" thick 6 ply birch plywood (supplied and laser cut by National Balsa) I created my own. It will measure from a 0.790 down to a 0.680 in 0.010 steps. (NOTE: the 0.005" width of the laser beam was accounted for in the design, so the holes are very accurate.)