I've heard this in the past that a bigger spur will give more torque because it has more leverage on the axle and that a smaller spur will be higher top speed due to the smaller mass of the gear. Personally I think the torque theory cancels itself out. I'd have to do the math to figure it out for sure. Think of it this way, yes a larger spur is going to have more leverage on the axle but to keep the same roll out or gear ratio means going to a bigger pinion as well, and the motor is going to have a harder time turning the bigger pinion. My guess is that the 2 cancel each other out.
On the flip side a case can be made that a smaller spur will give you both more top end and torque due to it's lighter mass makes it easier to spin. But then the same question comes into play...does the effect of the reduced mass outweigh the reduction of leverage from the gear.
Even if they don't cancel each other out, any gain would be so minor as to be nearly indetectable...Anyway that's my theory
