What I just recently used was an 18" steel ruler with 2 adjustable square heads on it. I use the square heads like a caliper to measure the overall tire width. Then I lay a straightedge (a 24" scale, straight 2x4, straight piece of metal) across the tire and measure from the bottom side of the straightedge down to the edge of the rim (about where the tire bead would be). Take this measurement from both sides of the tire, add them together then subtract that total from your overall tire width and it should get you close enough to know the rim width. On the backside of the tire, with the straightedge still laying across the tire, measure from the bottom of your straightedge to the rim mounting surface (where the rim mounts to the axle). Take that dimension and subtract it from the previous dimension you got from measuring from the bottom of your straightedge down to the edge of the rim (on the backside). This dimension will be your backspace. Granted not everybody has these tools. I'm lucky. These tools are part of what I have left from 35 years of building machine tools.
