What is the combo going in ??
Bigger pinion = slower acceleration and more top speed.
Smaller pinion = faster acceleration and less top speed.
Smaller spur gear = slower acceleration and more top speed.
Bigger spur gear = faster acceleration and less top speed.
As for gearing suck it and see, start of with stock gearing and go from there. But you will need a ir temp gun to monitor temperatures. once you know what temps your getting adjust the gearing as follows.
hot motor + cool esc & batt = undergeared
cool motor + hot esc & batt = over geared
warm everything = just right
hot everything = way Overgeared
Try and keep the motor under 180f
Esc keep under 140/150f
And lipos keep under 120f
That's the higher end of the scale, ideally all temps should be less, a warm setup is a happy setup.