[Graham]

68mm wide BRM P139 with 14mm tyres on...hmmm, not sure that's realistic.

Hi Kevan,
in order to make this a reasonable realistic event with minimum number of Classes, you have to make compromises. According to the Classic Slot Car Racing Association rules most of the cars post 1970 -1988 are realistic and specify 14mm Maximum tyre width. Only the type of cars you refer of the 1966-1970 era are specified with a Maximum 12mm tyre width. Similarly most 1970-1988 cars have a maximum overall width of 68mm. Again only the 1966-1970 cars are restricted to 60mm.

These are Maximum tyre width and overall car width rules, so if you want to be absolutely to scale there is nothing stopping entrants using 12mm tyres and restrict their cars to 60mm overall width for the BRM P139 or similar cars of that short era,  should you of course intend to enter one....