[Chris Doughty]

LRP 6.5 - 20/84 (if I had to use an 81 spur I would use a 19 pinion)
LRP 7.5 - 21/84 (tooth lower with the 81 spur)

I like the 84 spur because it allows the motor to be in a more 'normal' position compared to brushed setups.