I've been searching a bit about this but couldn't find anything about it.
Is there a "golden rule" when it comes to the relation between wheel diameter/wheelbase/track width?

All I could find was info on "real" cars but nothing about RC.

if it looks good then its good. Thats the golden rule

Oh come on now.....
Surely there's more to it than that.

Unless all the factory built RC cars were just slapped together and the manufacturers/designers said "That looks about right"?

Not really... the longer the wheelbase and the narrower the track width, the more stable the car will be in a straight line. The wider the tack width, the shorter the wheelbase, the easier it will be to get the car to turn. There's more handling factors, weight distribution, suspension damping and steering geometry, but I'm not aware of any "special formula" for wheelbase versus tire/ wheel diameter... never heard that one.

wider is better

Well, they do make an attempt to make it scale somewhat. So take the wheel size and figure out what scale size it is compared to 1/1 scale. Basic general wheelbase is about 100-120" and track width is in the 65" range. Divide these numbers into the scale size of the wheel. For instance, if you figure your wheel is about 1/6 scale the wheelbase of the car would be 18.33" and the track width would be 10.83" approx.

See this is the sort of info I was after.

Thanks.

hey, we cant just give you an answer so fast and easy, gotta make you work for it a bit

Haven't you been following my threads?
I think I work pretty hard....