In my previous post, commenter Jacob added that a human can travel more efficiently by bike than by foot, which shouldn't surprise anyone who's ridden a bike.
He ran the following calculation in Google--(31,548 Calories / gallon / 563 Calories / hr * 12 mi / hr) in mi / gallon. That is, a 155 lb person cycling at 12 mph will get 672 mpg. At 10 mph that person will get roughly 747 mpg.
I'm pretty sure this can't be as good as it gets. Anyone know of any vehicles that are more efficient than bikes? I imagine some engineering students have worked on this problem and outperformed ye olde bicycle.