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.
While buses clearly are efficient on a per passenger basis, the reality is that aside from plane travel, most of the others could probably be dramatically improved by increasing load factors. If more people road on trains they could probably approach the numbers of the buses without much difficulty. Commercial aircraft are already probably pretty crowded most of the time, and probably don't have much room for improvement. Cars could definitely be improved if more people car-pooled instead of riding solo everywhere they go, although it's unlikely they could get to the mass transit numbers unless most of the large trucks were replaced by smaller more fuel efficient vehicles. The full report is available at the Read link.
[Source: American Bus Association]