how to work out what gearing to use

[HootMaRoot]

got my motor mount from here a few weeks ago but i am still awaiting on my tekin RX8 and a 4000kv motor and 1 3S & 4S lipo to test out

but what i amnt sure about is how to workout what size pinion to use

atm it has a 46t spur gear

also dont know the ratio in the front/back diff's but not to sure if i realy need to know that