[E)(pul$ioN]

Heres something from the Tips & Mod section of this site:

This set comes with three different gear ratios, consisting of an axle (without tires) and a matching colored gear. You cannot mix the gear colors or the car will not run due to gear binding.

The gear colors are as follows:
Blue - good acceleration but not much top end. Good for climbing and technical tracks.
Orange - fair speed, fair acceleration. A good all around gear. This setup comes standard with all cars except special editions.
Teal - somewhat slow acceleration but great top end. Good for racing or wide open spaces. This setup comes standard with all special edition cars.

TinyRC: What do you mean by mixing the gears up?