[CMOTD]

Raising the gearing ratio by fitting a 10 tooth pinion will give slightly higher top speed at the expense of acceleration and braking, the benefits depend on how long your straights are and how many twisty bits you have. In any case the older SCX motors are so gutless that it is doubtful if you will achieve much.

A rough guide to gear ratios, pinion/contrate:
9:27 gives you good range of speed and pulling power and braking.

8:27 adds a little more pulling power and braking but reduces top speed. (for mildly twisty circuits).

8:28 adds yet more pulling power and braking but reduces top speed even more (for very twisty circuits).

10:27 gives more top speed and less braking and pulling power (for long straight circuits)

10:24 gives walloping top speed but hardly any brakes or pulling power (huge straights and hardly any curves on your circuit).

It is really just like your full size car, a low gear will scream you up to 50mph but then you hit the rev limiter and it won't go any faster while top gear will push you past 100mph but will take forever to accelerate from 20mph.