[Mick Kerr]

Sorry Savage but I must disagree with your sizing list.
The diameter of a car wheel is quoted as the diameter of the bead on which the tyre locates not the overall diameter of the wheel.
The depth of the bead which holds the tyre onto the wheel must be added to the wheel diameter to get the overall visible diameter.
The bead on most wheels is about 5/8 to 3/4 inch depending on the material the wheel is made from.

   
This diagram should make what I am saying clear.

If we assume 3/4 inch bead we must add 1.5 inches to the quoted diameter of any wheel to get the visible outer diameter.
This give a revised list at 1/32 scale as follows;

20 inch = 17.1mm
19 inch = 16.3mm
18 inch = 15.5mm
17 inch = 14.7mm
16 inch = 13.9mm
15 inch = 13.1mm
14 inch = 12.3mm
13 inch = 11.5mm
12 inch = 10.7mm
10 inch = 9.1mm

These have all been rounded to the nearest 0.1mm

So to take an obvious example
A Formula 1 wheel must be 13 inch diameter but it's visible wheel diameter will be approximately 14.5 inches diameter.
At 1/32 scale that is 11.5mm, not 10.3mm.

There seem to be a number of Slot Car manufacturers that don't understand this which is why many cars are fitted with under scale wheels.
I seem to remember having to explain the above to Andi Rowland a few years ago.

Cheers.
Mick.