Ok this is what I ended with about the gearing ratios.

I'll be running a 10.5 motor in blinky on a small technical track with medium worn tires to begin with.

With little help I was considering a 60mm to 55mm rollout (tell me what do you think). I considered the wear of tires and chose pinions so that I can get 60mm rollout when tires are new (45mm diameter) and 55mm when tires are used (42mm).

The Corally has a 98t spur, the AE a 96t spur and the CRC a 80t.

I used this site http://www.gearchart.com/index.cfm?f...n=chart.create
and ended with a 41t pinion for the Corally, 42t for the AE and 34t for the CRC.

Just tell me if I'm doing something wrong

There is a last question I try to figure out:
Is there any advantage of running a smaller spur with a smaller pinion rather than a bigger spur with a bigger pinion when both give the same rollout?

Do you run the stock spurs on your cars? Or is it better to run a diff with more balls (with xenon vss spurs for example)?