Hi there, bit difficult to help you pick apinion without knowing what turn of motor you are using and knowing how big the track is. But I run a 21 tooth pinion on a 76 tooth spur gear on a Dynamite DPS 6.5T motor (that gear ratio is for a straight length of roughly 25m). If you use a higher turn motor maybe go up on the pinion size (so 7.5 go to a 22 tooth pinion) but it does very much depend on the length of the straight.
All good model shops should carry pinions, get a small range so you can go up or down depending on whe you are running.
Hope this helps to get you going.
Chris
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