what pinion will suit a 3900kv motor

[danny the elfman]

Hello, i have an anmsann madmonkey which im running with a 3900kv motor, but its very slow compared to the other cars, the speed of my car i guess is around 30mph, but the other racers go around 35-40mph, maybe even 45mph. I will be upgrading the motor later in the year, but for now i just want a bit more speed. so whats the largest or best pinion to use, ive got upgraded idlers on order so there no longer a problem (the highest the pinion has ever been at is 21t) . Thanks for reading

[danny the elfman]

ive added some more information to help