As you may or may not know, brushless motors are a different breed of motor compared to brushed motors. They produce a considerable amount of torque, and that being said you'll need to covert the motors torque into speed. The only way you can really do this is by running a smaller spur gear (78) and bigger pinion (38) tooth. I've tried running a big spur (96) and (55) pinion, but as your rollout changes, so does your pinion. When the tires get smaller, you're rollout gets smaller, so you'll have to adjust for this with a bigger pinion. I've had situations with a big spur and pinion where my motor couldnt move any further back and wouldn't let me run a larger pinion gear.

Basically, bigger spur and pinion limits the amount of room to play with on you motor mount.