What Pinion you use is depended on what motor you have. Your motor has a kv rating like my AON as a kv of 3500. This means for every 1 volt I put in I’ll get 3500 rpm. This is where your gearing comes into play.

Example:
Motor AON 3500
Battery 3s
Pinion 15t
Main gear 150t
Motor Efficiency 80%

3500*11.*15/150*8=3108 this would be an approximate head speed