they make almost any gear size in any pitchs.. here http://www.robinsonracing.com/
and 30 minutes is a long time to be bashing . Id add a fan and shorten my run times down to half of that 30...
id start with 2 teeth under ur present gears ,,as in a 19t
temps let us know a little more
motor turns?
battery type?. 2cell or 4 or what?
tire size?
run times