VNelson: Nice first post. It should be good to have some new insight on things around here. Your breakdown was easy to understand, and at least in my opinion, warrants some serious thought. You could of course complicate things by factoring in the fact that Snickers aren't free energy, and that they consume energy during their raw materials collection, production, and transportation.
Otherwise, it was an interesting read. Perhaps you could look at a less energy intensive item, like home-grown fruits?
If you are talking vegetables though, then yes, they are for the most part pretty low in calories. Fruits are usually loaded with sugars and other carbohydrates though, so they can hold quite a few calories. Vegetables mainly just have vitamins and minerals though. Avocado can be pretty fatty, but your average green pepper, tomato, and carrot are relatively low in calories.
Searching for calorie dense foods, it seems that soybean oil (generic) will get
your mpg back up there. A bottle sells for $2.50, 1 T is 120 calories and there are 96 servings per bottle for a total of 11,520 calories. That works
31000/11520*$2.50= $6.73 per 'gallon' and a factor of $6.73/3 or 2.24
1350/2.2 = 614 bike mpg
310/2.2 = 141 walking mpg
All in all, though, not sure you could actually eat the stuff in those quantities.