Try the FDR calculator at the bottom of this page:
http://driftmission.com/tools/
Countersteer Ratio/Final Drive Ratio is your internal gearbox ratio which as standard is I believe 2.6
If you've got an Android phone I use a free app called Gears from Nor-cal Hobbies, there are loads kicking about though...
PS - If you're down on top speed I found the biggest difference was changing/updating ESC software as you're geared high anyway