Just to mix things up a bit, take a look at this car from the 2008 Eco-Marathon, registering at a whopping 7,148 MPG (via Autopia):
Looking like one of those space caskets from Star Trek, the single-seat racer you see to the left, constructed by a team from the French technical school St. Joseph La Joliverie and dubbed the Microjoule, traveled an amazing 7,148 miles on a single gallon of fuel (in other words, almost ten miles per teaspoon).
Of course, at some point the notion of fuel economy becomes arbitrary (and absurd). I probably used more gasoline walking to the library today (amount of food energy I had to eat to get here X the embodied energy of my food).
But more power to the engineers involved. I fully support their employment in the auto industry.
Here are two of the other competitors:
Photo Credit: Shell