The^Thief has hit the nail pretty well on the head, but here is another factor many people ignore in their concideration of drivetrain losses. Tires.

Tire distortion sucks up a good chunk of the energy we concider part of 'drivetrain losses'.

Just think about how hard it is to push your car across a smooth flat surface. You can't blame all that drag on the gears and lubrication in your tranny.