Originally Posted by slots

The servo is fine - I've used the same one for about 2 years.

Rather than using just gear ratio, roll out is a better measure of gearing when the tire diameter can vary. The calculation is like this:

Tire Diameter X 3.1416 (PI) X pinion / spur = roll out

Sample 1/12 rollout for a stock motor:

1.8" rear tire X 3.1416 X 30 tooth pinion / 96 tooth spur = 1.767 inches of travel per motor revolution.

I carry a calculator in my tool box just for this purpose. You could also use a spreadsheet to do a chart - maybe somebody can post one.