If humans utilized gasoline for energy instead of carbs, fat, protein, and such, how much gas would they need to drink a day to keep a balanced weight?
Assumptions:
2000 Kcals are required a day
Unleaded
Regular
According to this page, 1 gallon of gasoline equates to about 1.3 x 10[sup]8[/8] joules of energy. 1 food Colorie = 1000 calories = 4,184 joules. 2000 Calories would be 8.368 x 10[sup]6[/sup] joules, so you could get this by consuming .064 gallons of gasoline.
Of course, you’d probably get sick and die first. 
Well, that was a mess. Let’s try this again. 1 gallon of gasoline equates to about 1.3 x 10[sup]8[/sup] joules of energy. 1 food Colorie = 1000 calories = 4,184 joules. 2000 Calories would be 8.368 x 10[sup]6[/sup] joules.
That’s242 cubic cm, or 1.02 cups of gas.
Sheesh. And I forgot the link to this page where I got the conversion factors from.
Imagine how cool that would be. Get up in the morning, drink a cup of gasoline, and you’re good for the day. Of course, the depletion of our fossil fuels would mean our imminent starvation and extinction, but I think that’s a small price to pay for such convenience.
OTOH, taking your date for a romantic meal at the Exxon station just doesn’t cut it.
“Try the 91 octane. The Techroline makes it truly superb.”
The answers given so far are upper bounds. You can’t use 100% of the calories that you eat, though (google on “bioavailability”), so the actual yield would be slightly less.
Mmmm… Gwondonaland Private Reserve 330,422,901 BC.
That was an excellent year.
For comparison with edible foodstuffs, you’d have to choke down 1.25 cups of butter for those 2000 calories. 
Imagine getting on a crowded elevator, the doors close, then a cloud of putrid exhaust rises.