Originally Posted by niznai

You need to use the same pitch pinions, but higher number of teeth (larger diameters), not smaller. This is the whole idea of having smaller spurs as those in the "speed tuned" gear set.
Together with the "speed tuned" set of gears for these cars comes a diagram that shows how the mounting positions of the motor change to accomodate for their smaller diameter. You can now use larger pinions which will give you more speed.