TREAD DIAMETERS are a fairly good guide to wheel size, but remember that due to apparent tyre thickness varying from wheel to wheel by 3 to 5 inches, you will need to refer to the diagrams to find the internal diameters and to see the machining allowances on the castings.

Finding the right casting size if you know the apparent tread.

Look for the casting i/d which matches your model diameter on the tread (the normal nominal o/d), less TWICE the apparent tread at your scale.

Example: 5'0" nominal dia. wheel with a 3½ inch apparent tread at a scale of 1:32.
Model dia on tread = 47.6 mm, twice the apparent tread = 5.6mm, so your target casting internal dia is 42.0 mm.

There is some variation in the castings, but the measurements are usually accurate within ± 0.1 mm.