[TonyM]

It really depends on the track size. Go with the age old idea of gearing to reach top speed at around 2/3rds of longest track section and/or a motor temperature of around 160 Deg C after 5 minutes hard racing.

With a 13.5 motor you probably will want to drop the spur size from the standard 87T kit. Otherwise the motor housing screws will probably not fit into the plate slots.

I would suggest playing around in the range 81 spur with 28-30 pinion. Even then it may be a tight fit.

Or, if you want to push your motor much further back a 79 spur with a 27-29 pinion. Or even a 75 spur with 25-27 pinion