One neat fact (among many) I learned from xkcd's "What If?" is that gas mileage ultimately has units of inverse area (i.e., 1/length^2). This means a fuel economy of 20 miles per gallon is equivalent to how many inverse square feet (1/ft^2)?
789,888 (766,191 to 813,585 acceptable) (edited at 6:29 am, September 2, 2015 (EDT))
Original Answer: 14,117 (13,694 to 14,541 accepted)