Originally Posted by
Brian McGreevy
For even relatively small on-road tracks, you'll want to be between 3.5-3.7 FDR, depending on the timing your motor needs to run its best. I'd recommend a spur of between 86 and 92 teeth. Pinion sizes between 46 and 52 teeth.
The instruction manual has a chart in the back that shows what will fit.
If 4:1 is what you want a 100/50 looks optimum to me as it gives you the most up down range.