Originally Posted by dbowen

The 3 gear is really only better for high bite tracks because it reverses the rotation of the motor to pitch the car forward on power vs towards the rear thus creating on power traction, so the 3 gear takes away on power traction and the car stays flatter in the corner; and for stock where there is one less idler gear and less rotating mass and stock doesn't have enough torque to really notice the loss of on power traction and a lot of times you want to free up a stock car which is why it is better.

The 4 gear will still be way better in mod for medium to low bite tracks and therefore most tracks around.