If you are looking for the most efficient form transportation propelled by an internal combustion engine, you won't find it at a Toyota or Honda dealership. Or any other car dealer for that matter. In a study funded by the American Bus Association, it was found that the most fuel efficient transportation in the United States was by motorcoach. Based on mileage and passengers in 2004, highway buses achieved an average of 148.4 passenger miles per gallon. That's more than double achieved by intercity trains which achieved 74.1 passenger miles per gallon. Airlines managed 40.9 passenger miles per gallon, while cars came in last at 35.4 mpg.