I wondered the same thing. In fact, I saw just the car I wanted while on vacation in VA. I didn't get a picture, but it was a little Citroen AX. The plates on it said "Diplomat". The fender said that it had a 1.5 diesel in it. Here's a pic of one from Wiki:
The one I saw was the same color as this one. They have plastic panels in non load bearing areas, a drag coefficient of 0.31, and a very light weight of just 1,411 lbs!
Wiki says they can achieve 80 mpg
without hypermiling. [...per 153 ounce Imperial gallon, or about 65 miles on one of our 128 ounce gallons].
Perfect! Why can't I get one in the US?