[Gielsy78]

Okay, so if we take as an example some Slot.it Ø15.8 x 8.2 mm or NSR 16x8, something in that range? 

Maybe not expecting a definitive answer, just some rough idea. Given track is measured to the tyre centre line on each side, and the screw mounting point for most wheels would seem to allow the axle to reach that point, I'd expect the required axle length to be more or less the same as the track. However, i can see cars listed with 55mm track and 48mm axles, for example, so that clearly isn't the case.