I've never driven anything with more than 20 forward gear ratios. Big Jim said it perfectly:
In reply to:
A lot of gear selection has to do with matching the power curve of the engine with driving conditions. sometimes even a slight change in gearing can make a big difference in having the engine in it's best part of the power band accelerating out of a curve on the track. That is where experience and experimentation comes in.
If you have a motor with a flat torque curve, you need less gears than if you have a engine with a sharply peaked torque curve. So number of gears and ratio's depend on the length of the track, the sharpness of the curves, weight of the car, hp and torque curves of the engine, rolling resistance and aerodynamics of the car.
Look at Indy cars. They all look alike. Yet someone wins and someone comes in dead last. Small differences in gearing, engine and of course skill, make the difference.
Ratio choosing would depend on a lot a factors. Do you have a specific question on which vehicle/motor/condition you would be selecting ratios for?
