Just thought I would through this out here.

www.comeseethis.com

There are a lot of sites out there that will give you a gearing chart so you can will know what final drive ratio a particular pinion and spur gear will get you, but how do you know what is the best FDR for your particular application? This site wille help you figure that out. Pick your car and motor and it will tell you what the motor manufacturer recommends for a FDR, and what pinion and spur gear you can use to get it.