I noticed Bullet has a Gear Constraint in source, however it was not exposed to Blender.

Ive implemented the Gear constraint in blender. it allows a simple way to set up a relationship between two objects rotation ratio.

I have 1 example and some screen shots and currently creating documentation.
http://wiki.blender.org/index.php/User:Levon/bullet_gear_constraint.

The example sets up 3 different gear relationships using gears of different teeth ratios. and a driver to allow for a change in speed.