My diesel f350 gets about 18 mpg as opposed to a gasoline burning F150 that got about 15 to 16. Its not an apples to apples comparison but if you consider that the f350 is a much larger truck it may be applicable.

I have no faith in human perfectability. I think that human exertion will have no appreciable effect upon humanity. Man is now only more active - not more happy - nor more wise, than he was 6000 years ago. Edgar Allan Poe