Light On Dark Water

I suspect that most of us are either (1) the sort of person who is willing to drive somewhat out of his way to save 5 or 10 cents per gallon on gas or  (2) the sort of person who suggests that the amount saved may not cover the cost of getting there, or in general be worth the trouble.  I'm definitely in the second group. But while I am enough of a rationalist/math-sci personality to have the thought, I'm not enough of one to actually sit down and work it out. But the guy who draws xkcd is. Notice the rollover text.

I sort of liked this recent xkcd, too, though it's not funny. You've seen those decals people put on their rear windows with stick figures representing (I assume) the family's assortment of parents, children, and pets?