This is what I am largely basing my hypothesis on what is important in efficiency:
http://ecomodder.com/forum/showthrea...tml#post181816
After the X-Prize, Edison2 which got ~102MPGe Combined and 129MPGe Highway, weighing 830 pounds powered by an ICE - they then put in a electric drivetrain and the weight went up to 1140 pounds - but it gets 245MPGe.
The Allert Jacobs streamliner is another reference point:
214 mpg with DIY aerodynamic fairings on a Honda 125cc motorbike
~90MPG stock weighing about 151 pounds, but with the aero shell and taller gearing and weighing 231 pounds, it gets 214MPG.