Take the number of teeth on the spur gear and divide it by the number of teeth on the pinion gear. The higher the number, the more low end you'll get but sacrifice the topend speed. The lower the number, the higher the top speed but the low end torque will be sacrificed. If you're in a place that has a lot of straights top end will be good. On a technical track with twists and turns, more lowend will be needed to get out of the turns quicker.