Larry Elie's mail of tractor efficiency made me think this info on mechanical drive efficiency might be appreciated. All from memory, so if there is something I have forgotten, ask about it and I shall look it up.
First, the efficiencies I will list are based on a 1 to 1 ratio. As the ratio gets larger, the efficiency drops, although until you exceed somewhere between a 3 and 5 to 1 ratio, the loss in efficiency is not much. Also some devices, such as worm gears, which are bad enough in speed reduction mode, are sometimes so bad they won't even operate in speed increasing mode. They lock up.
Remember that these are general rules and each specific application will have an efficiency associated with a clean and proper installation of new equipment. As the equipment wears, efficiency can change, and while in some cases (timing belts) the loss will be minor, in others (chains) the losses can be severe.
1. Chains properly tensioned and lubed: 99% (Think of how many inroads have been made into chain replacements on bicycles.) (Also the famous Sturmey Archer 3 speed bicycle hub can have an efficiency as low as 80% in some speeds, although that is in part due to the use of plain bearings.)
1A. Dry and/or dirty and/or worn chains: Less than 75%
2. A, B, C, D series V belts: 95%
3. L series belts: A little better than A, B, C, D series, but not much.
4. V series V belts: 96-97% (The GE large frame motor to transmission belt uses this series because it is the best of the V belts in terms of efficiency.)
5. Timing belts: 98%
6. Poly V belts like the serpentine belts on newer automobiles: 98%
7. Spur gears or helical gears: 97-98%
8. Bevel, spiral bevel, hypoid, and other gear sets that turn 90 degrees: 90-96% (It is no accident that many of the smaller front wheel drive economy cars use transverse engines. Eliminating the right angle gear set at the differential needed for any longitudinal engine installation and using a helical gear set instead helps get the fuel mileage up.
Hope you all found this interesting.