I was walking at the time, however. And surely, I thought, I'm more efficient than a big, heavy non-evolved car, even a Prius, but by how much?
How many miles per gallon of gas does a human get?
One answer is zero--humans obviously can't convert the potential energy contained in gasoline into kinetic energy. That's a boring answer, though.
How many miles per energy contained in a gallon of gas does a human get?
A gallon of gas contains roughly 132 megajoules of energy, 31,548,757 calories, and 31,548 kilocalories, or food calories, or calories, in common parlance (one food/kilocalorie, remember, actually contains 1,000 calories).
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!).
The friend who noted this also wondered how many miles per Big Mac a car gets. (Big Macs, he reasoned, are a "generic unit of consumption." Indeed.)
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.
* A friend ran the numbers from another source that assumed slightly more efficient walking and got 406 mpg, so 400 is a safe approximation, it seems, though we both concede that we might be way off or missing something key in making these calculations. If any readers try to answer the same question, add a comment to tell us what number(s) you got.
Also, these numbers vary widely according to how much a person weighs and walking speed. Running, for instance, is relatively inefficient, which I suppose explains why people jog to burn calories.