New RX-8 In My E-Maxx
 

I anticipated keeping the twin Titans in my 3903 for at least a year, but here I am pursuing my long lusted-after Tekin RX-8; I chose the truggy 2D 2000kV motor, paired with an 18-tooth Robinson hardened pinion and stock 68 spur.

With stock 7-cell packs, this thing hauls and runs very well overall. But I've noticed in the 90-degree Chicago heat the ESC runs very cool but the motor is heating up more than it did when it was cooler out; not hot enough to prevent me from keeping my hand on it, but pretty warm; maybe 140.

Suspecting it was a little undergeared I threw on the optional 62-tooth spur, giving it a slightly higher ratio over stock; the motor had no trouble and runs a tiny bit cooler, but would there be any harm in going with a 20-tooth pinion on either the 62 or 68 spur? This would gear it up in hopes of putting tailoring the gearing to the motor's available torque so it could run cooler.

On my E-Maxx, I run 19/65 with a 2039kv 2-Pole Motor, gets around 168-171 fresh off a hard run on 5S.

You should be fine with that gearing on a RX-8. Keep it under 170 and you are fine.

If you are running 4S, consider moving up to 5S, as higher voltages are more efficient.