Just some info about spur gears. If your like me who run LRP BL indoors you may need to gear the motor around 9.0-10.0 sometimes, in that case you need a BIG spur gear. I've tried some different sizes and the biggest i would use is 114T, a 115T may be flush with the underside or protrude just a bit.
However, to use a bigger spur than 107-ish you MUST enlarge the hole in the bottom chassis that the spur goes through. It has to be extended a couple of mm's to the rear of the car.
With a 21 pinion and 114 spur you get a FDR of 9.9
I hope this were to any help.