These cars usually have a one cylinder engine with about 0.05 liter displacement that runs at full load when running. The special body and wheels allow them to coast with the engine off most of the time. They go slow (about 20 MPH) to keep wind resistance low. I know I've seen some details on the web. Try searching for "supermileage" or "3145 MPG".
Here are some details on a super mileage car engine:
Cal Super Mileage Vehicle Engine