Want to better understand gearing

[lincpimp]

Also keep in mind that having more teeth on the pinion will help with how well the spur and pinion mesh.

Pretend you have the desire to run a 3:1 ratio, you can accomplish that with a 10t pinion and a 30t spur, but a 20t pinion and a 60t spur will run smoother, as the teeth angles are better on the 20t and more teeth are in contact with each other at any one point.

So a larger pinion with the same spur will increase your top speed, but place more load on the motor, esc, and battery.

A smaller pinion will provide faster acceleration, but less top speed. You can gear a system too low, and the motor temps will be high as the motor is revving too fast with not enough load. Not usually a problem, but say you are trying to run an 1/8 buggy with a mmm/2200 combo on 6s lipo and it is geared for 20mph... that will cause motor heat issues.

The opposite applies with spurs, smaller spur = higher top speed, larger spur = lower top speed.