Ok, here goes... Don't think I don't like expressions, but I'm just a big proponent of SDK's.

Assume you have 2 gears. Gear 1 is the driver, gear 2 is the driven. Load them both appropriately then select rotate X for both of them. Now hit the Key button in the SDK window. Select your first gear then rotate it 360 on X. Then select the second gear and rotate it -360 on X. Then hit Key again.

Now, set your keyframes on the X rotation of the 1st gear. Then go to the graph editor. Select the graph for the X rotation of the 1st gear then go to Curves > Post Infinity >Cycle. The animation should go on for ever and ever and ever....

To adjust the speed, just adjust the keyframes on Gear1, and the rest should follow.

Ok, so maybe SDK doesn't do EXACTLY what your expression does on it's own, but it does with a little help from Infinity Curves

Hope this helps some more.