ok, if you have 2s packs already... makes sense then.
best to get a range of pinions 17-19t to find what works best for you.
at least 2400kv motor is good.
bigger pinion increases current and therefore heat and it gets to a point where it too much for some of your components usually the battery. beyond a certain point it doesnt increase speed anymore and simply makes more heat which is totally inefficient. so we tend to start with smaller pinion obviously and incrementally work up.