A very nice site that explains a simple way to increase the gallons per mile of a Geo Metro, simply by reducing drag:

This car, originally, was a joint venture between General Motors and Suzuki, which delivered as much as 58 miles per gallon. 49 hp is all it takes to get a person to work safely and comfortable. I reworked this car to get even better mileage. It was getting 75 mpg during the 2 years I drove it. The bottom line is, with a 3-cylinder engine 12 valve variable cam, a small streamlined tandem-style could get even better mileage.

What, no roof-rack? I guess the real down-side is that when you cut away half of a Geo Metro, you are still left with half a Geo Metro. But seriously, the problem I see is that the footprint on this thing is still four wheels and now just has a lot of wasted space. That would not appeal to someone looking for a smaller vehicle, and it lacks the storage for someone who needs a larger vehicle. High marks on drag efficiency and gallons per mile, but who would find it useful beyond these advantages?