[Kevan]

So back to first principles...why would you want to change the pinion?

To change gearing to suit your particular track.

I hear what you are saying, but I always thought these Formula XX cars were meant to be a fixed standard spec 'class' that would allow inter-club competition

and how many are going to be used for that? probably a minority.  There's at least one in our club and he has zero intention racing elsewhere