Weight is not as critical as drivetrain efficiency or aerodynamic drag. The aero of the Jacobs 125cc motorcycle is about 80 pounds heavier than stock -- that is more than a third heavier.
Taller gearing and good aero = more than twice the FE.
Also, the
X-Prize Knockout Round provides data that shows that drivetrain efficiency is the most important factor.
Aero drag is roughly 50-75% of the total load much of the time at normal speeds. And it is a total loss. Weigh means kinetic energy, so you can make use of it by coasting, so you only lose this energy to drag, or if you have to use the brakes.
Back on topic: I think the Prius (3rd gen) Atkinson cycle engine is up to 38% efficient? And some TDI diesels are 42% or maybe 44%?
Critical to raising the average efficiency is warm up time, and idling. Another issue is how to get the required electricity (for gasoline) more efficiently -- maybe by braking with the alternator? (I think Mazda will be doing this?) Reducing parasitic friction within the engine -- electric valvetrain maybe?
What about some of the new rotary designs? The geometry of a crankshaft can be improved, that's for sure.