whats the highest gearing I should use?

[shaunjohnson]

well i run 18/65 gearing on my E-revo for racing conditions and temps stay around the 120 mark on 6s.

so i guess you could try a 20t pinion and move up to a 22t.
basicly play with it till you find a sweet spot.
then after you find the top speed sweet spot i recomend gearing down just a tad to allow you to have a bit of fun (high load acceleration, wheelies etc)

this is all IMO so it's really up to you.
there are many many other more experienced people on this forum who might chime in later on
shaun