There are a number of slides, each one is for a different spur gear. The left hand column is tire dirameter and the top row is spinion size. The middle of the matrix is the inch rollout. For tight tracks you want to be in the 0.7-0.85 range, for fast tracks from 0.8 to 0.95 ish. You need to do this from experience and what kind of motor you run. GM3s will be geared much shorter than a P2K2. Depends on where the power band is on the motor.

Anyway, measure your tire diameter, this will give you the row to look at. Then pick the rollout you want to run. You may need to look at different pages to pick the right spur gear. Once you find out where you want to be with the spur you want to run, then pick the pinion to match. With newer grey dot rears, they will wear very slowly. You can probably stick with the same pinion and spur for one race day. If you true between races, then you'll need a different gear combination to stay at the same rollout.

These charts let you run the same rollout for different tire sizes. So as your tires wear, your car will accelerate the same and have the same run time.

This is one of those details that you need to pay attention to in order to be competive on 1/12 scale class. Oh, yeah, run consistent lines and don't crash, that helps too. LOL

Sands