Gearing

Firstly, gearing is more than just spur an pinon size. We need to take into account all the reductions or multipliers from the motor shaft to the axal shaft. This is what is called Final Drive Ratio or FRD. This ratio is comprised of spur, pinion and trans ratio. 

( Spur / pinion ) x trans ratio = FDR

Tran ratios

Here are some examples of trans ratios:

 Traxxas = 2.727
Losi 22s = 2.43
TA B6 = 2.6

Apollo = 2.6

Each of these trans ratios are different so the same pinion and spur combination will give you different FRD results depending on the car.

For example

A traxxas car with 86t spur and 20t pinion will use:

(86 / 20) x 2.727 x = 11.726

So the FDR is 11.726.

Motors

Now each motor will like a range of FDR to work at its best.

 I don’t have the most exact info here but:

4.5t like 11 to 13 FRD

 3.5t like 13 to 15 FRD

 4 pole 6400 kv motors 7.5 to 9.0 FRD. 

Ghost 1408 7000kv 9 to 10 FDR

Depending on many factors your FDR will change. Never think of gearing as just one option, you need to consider that you will need to work with multiple FDRs at any point in time. 

For No prep racing FDR selection can help is so many ways. 

Lower FDR numbers will help when traction is limited. lowering the FDR removed the motor mechanical advantage making it harder to spin the tires. 

Higher FDR numbers will help when the track is sticky and you need to get out of the prep. Higher FDR numbers give the motor a mechanical advantage.

So, to help work out what best pinion to use, punch your numbers into the equation and aim for the above-mentioned FDR.