[DampRobot]

I usually use these (highly inaccurate) estimates, based a little on research, and a little on experience:

Belt reductions: about .98 efficient
Gear reductions: about .95 efficient
Chain reductions: about .90 efficient
Planetary reductions: about .8 efficient
Single lead worm reductions about .6 efficient

Of course, as T^2 said, these depend heavily on a lot of factors. I've noticed (although not in the least bit empirically) that for reductions that are less efficient to start off with, misalignment, lack of lubrication, high speed, etc makes a much bigger difference in terms of efficiency. For example, it's much worse to misalign a worm gearset than a spur gearset in terms of efficiency, and belt reductions are much happier at high speeds than chain reductions. Of course, I don't have any numbers to back these assumptions up, so this may be useless to others.

The process I usually use is to assume that these efficiencies are just multiplied by the free speed of the motor to get the adjusted free speed, however, that isn't the real way you do it. I'd be very interested to hear what the actual meaning and use of efficiency is.

Really, what you want to do is to use some reasonable numbers to estimate how your system will perform, and leave enough flexibility in the system that you can regear if necessary. Maybe you won't need to redo it on all systems, but every year we've needed to regear some stuff, and not because we didn't do our math. Efficiency (like friction) is something that's very hard to analyze.