This is a derivative of http://www.thingiverse.com/thing:3575.
I made this to explore different ratios for my Vaterra Ascender.
With Openscad, it's pretty simply to create whatever spur-gear combinations you want.
measure distance between the centers of the spur and the pinion you have now
set TEETH_1 and TEETH_2 as you wish
- modify CIRCULAR_PITCH_DEF until gears mesh correctly; you might want to modify SPUR_ROTATE_DEGREES so that you can see the teeth mesh