The short answer is that fuel efficiency is a much lower priority to motorcycle riders than speed, acceleration, road comfort, and engine smoothness. Bear in mind that a bike now getting 50 mpg or less has anywhere between 80 and 110 horsepower-- or more. Nothing short of a dragster will accelerate more quickly than a modern sport bike, but this power comes at a cost. The engine must be relatively heavy, and the gas consumption high. A special gasoline efficiency project vehicle, however, is going to have to make some sacrifices.
As I mentioned, an athletically trained human being on specially faired bicycles can pedal at over 70mph-- albeit for a very short time. Cruising speeds, however, can be well over 40 mph for extended times. This speed is all possible with a power source that never exceeds 1 horsepower, and even comes close to that for only brief periods. Most of the time even trained athletes can produce only 1/2 to 2/3 horsepower, or something less than 600 watts.
These bicycles give some idea, then, of what is possible even for a very weak power source through reducing wind drag and keeping a vehicle light. Such measures can be taken with materials no more exotic than fiberglass and aluminum. One constructs a fairing to cover the bicycle, and a frame sufficient to suport a rider and the fairing will weigh in at less than 30lbs.
The gasoline requirements for a small combustion engine that delivers comparable power are very low. Even an engine that produces 3 hp or so, an engine that triples the maximum human effort, would use very little gasoline. Yet this power would be more than sufficient to drive a light, faired vehicle at road speeds. A vehicle something like a two-seated recumbent bicycle, with a fiberglass fairing and an aluminum frame supporting a small engine would easily double the fuel efficiency of a small motorcycle weighing several hundred pounds with relatively poor drag characteristics. That already would put this cheap vehicle at well over 100 mpg. Without having calculated anything carefully, I'd say 150 mpg would be easily reachable.
The exotic materials on this experimental car help, but the lion's share of the result can be produced with conventional materials. The primary requirements are a light vehicle with very good drag characteristics. This is possible without the need to visit the materials science lab.
Cheers.