[Jeffrafa]

The Drivetrain Efficiency is kinda just what it sounds like it would be, the efficiency of your entire drivetrain, both gearbox and drive system combined (chains, belts, gears, etc that connect the motors to the wheels). Personally i would think .65 is a little on the low side, but i must admit i havn't done too much with gearbox design and building, its never bad to stick on the safe side though. As far as coming up with an overall value for your drivetrain efficiency you need to multiply together the efficiencies of each stage of your drivetrain, for example if your gearbox consisted of two sets of spur gears that were each 95% efficient and you then had a chain with an efficiency of roughly 95% also then you would multiply .95 x .95 x .95 = .857, or roughly 86% efficiency. It mostly depends on the types of gears you use in your drive system and how much your drive system consists of.

Some rough efficiencies for different types of components are as follows:

Chain & Belt ~ 95% - 98%
Spur Gears ~ 95% - 98%
Bevel Gears ~ 90% - 95%
Worm Gears ~ 40% - 70%
Planetary Gears ~ 80% - 90%

Hope this helps
Also, please correct me if i may be wrong on something - its rather late to be posting :-)