Well that all depends on track layout but a good baseline (at least for me) is start off with a 75 (76 if using stock avid triad slipper) spur and a 28 tooth pinion. I think its a great balance between speed and torque. I wouldnt recommend any spur smaller than 72 or a pinion larger than a 32 tooth. anything more will bog the motor too much and the performance will really lack. not to mention your motor temps will be astronomical. I used to run 69/33 but now i run 76/29 and its dramatically better. Youre gonna have to mess with gearing til you figure it out for your track but 75(76)/28 is where I would start.