A 155 lb human walking at 3 mph will burn 246 kcal/hour, or 82 kcal/mile. Feed that human one gallon of gas in potential energy--31,548 kcal--and he'll have enough energy to walk for 128 hours. At 3 mph, he'll cover 384 miles; in other words, he'll get 384 mpg at that weight and speed.* That's seven times more efficient than the Prius, and 24 times more efficient than a Hummer H2.
It is worth noting that the Prius, say, can carry five people without seriously compromising its efficiency, and it can go well over 60 mph, and carries a big gas tank, consumes zero gas while out of use, and so on (people drive cars for a reason after all!).
As it happens, caloric as a Big Mac is, a car still couldn't get too far on such a diet. McDonald's says a Big Mac contains 540 calories, which translates to about 57 Big Macs of energy in a gallon of gas. By this measure, a Prius could get almost a mile per Big Mac, while a H2 could only get a quarter mile, and bit further on a quarter pounder.
link: David Archer via Welsh View
