in general:
for stock motors, you will need smaller spur + larger pinion in order to get a smaller FDR.
for mod motors, you will need larger spur + smaller pinion in order to get a larger FDR.

each car is different because the motor mount spacing is different.
for xray, if i use 48pitch, i find the upper limit of the total tooth (spur + pinion) is around 112 teeth.
eg: if i use 84 spur, the largest pinion i can fit is 28