Hey.
There is a simple gear creater inside the Physics Engine.
When you click with the "Joint Connector"-tool in one motor/hinge/slider/piston and hold Ctrl (control) -key and press a second motor/hinge/slider/piston these Joints connect to each other, a motor/hinge to a slider/piston would be a wormgear and motor/hinge to another motor/hinge would be a normaly gear.
With this trick you don´t need that much geometry in the physics model that slow it down.
I had attached a model with example of a motor to slider, in two ways. first, with physics geometry and a second with physics gear connecting that I explain earlier in this text:D
hope this maybe would be some help.!
/hpnisse