The 25 MPG Model T: Why haven't we done better?

The whole 25mpg Model T story started with an ad devised by the Sierra Club to embarrass Ford during its centennial celebrations in 2003, which unfavorably contrasted the 16mpg Explorer with Henry's flivver. Never mind Ford's own website claims the Model T got 13-21mpg (but of course, say the conspiracy theorists) the Sierra Club's 25mpg number is now accepted as fact by credulous media outlets.