VW has made a 1 cylinder engine, 2 seater car that gets 280mpg.
Volkswagen to Make Limited Edition of 1-Liter Car (282 MPG!) in 2010 : TreeHugger
It can be done, but the car ain't gonna be SUV sized like all us Americans like.
When I was running veggie oil, I once got 175mpg on the diesel fuel I paid for, does that count.