Originally Posted by SLOWINSLOWOUT

47-60 should cover you. The bigger the tire, the smaller the pinion needed for the given roll out. If you are just starting out, aim for 95mm roll out, this should get you going on the track, may not be optimum, but you can adjust from there. You will also need a good set of digital calipers to measure stuff with, tires, shims, springs, etc.

To calculate roll out, a bit of math is required. the formula is;
(3.14*tire diam in mm)/(spur/pinion)=Roll out

So, if your tires are 46mm, spur is 73, and pinion is 48 it will look like this;

(3.14*46mm)/(73/48)=roll out. solve in the brackets first;

144.44/1.520=roll out;

95.02mm roll out.

As the tires get smaller, the pinion goes bigger, approximately add one tooth to the pinion for every .9mm of tire wear.

For the same sized tire, the bigger the pinion, the higher the roll out.

Roll out is the distance in mm the car will travel with one revolution of the motor.

This should get you started.

If the motor is getting too hot, above 150 deg F, lower the roll out, and or lower the timing on the motor.