"There is a math illusion here," said Richard Larrick, a management professor at Duke University, whose research appears in the journal Science.
Larrick said most people think improvements in miles per gallon are all the same, where a 5 gallon per mile improvement would yield the same gas savings in a car that gets 10 miles per gallon or 20 miles per gallon. (One mile equals 1.61 kilometers, and one U.S. gallon equals 3.79 liters.)
"The reality that few people appreciate is that improving fuel efficiency from 10 to 20 miles per gallon is actually a more significant savings than improving from 25 to 50 miles per gallon for the same distance of driving," Larrick said.
Go read the whole thing. It's very interesting--and simple to understand.