[Graham]

To some extent I have been guided by the Classic Slot Car Racing Association Rules (CSCRA) on this proxy, and in fact the idea of the era I have chosen  is because the rules for the period 1950-1962 are remarkably similar across Sports, GT, Touring and F1 cars.
One issue I personally don't like is the Difference in wheel & tyre diameters (according to their specs 19mm front & 21mm rear), as apart from any slot car racing advantage, it just seems a little unrealistic to have smaller front wheels on a 50's or early 60's Sports or Saloon car than the rear ??
So initially I thought to split the difference at 20mm, then I thought well actually if 19mm is reasonably accetable then why not make it the Min limit for all 4 wheels on the car.  I have looked at the current cars in the Targa Florio proxy and measured approximately the wheel/tyre diameters of the 50's era cars. I think in appearance 19mm of some cars just looks about OK, some with 18mm look out of scale to me, above 19mm is obviously better but I am looking for a rule spec that "everyone" can work with.

I also looked at track clearance rule, but most of the cars currently seem good with that, so I think it should remain as it is.

Rear Tyre width CSCRA rules are 7.5mm for this era cars, so I allowed Max 8mm which I think is a reasonable max limit.