>As Europe has noted, there is a difference between miles per gallon and gallons per mile (or in most cases, per 100 miles). While it may seem like this is simply an issue of symantics, we assure you it is not.
As we noted before, increasing MPG works in a manner similar to the theory of diminishing returns. That is, going from 17-20 MPG is a much bigger step than going from 30-33 MPG. This article from RFF explains it rather nicely.
“Another nice feature of GPHM is that fuel costs rise directly in proportion to that rating. So, a 10 percent decrease in GPHM equals a 10 percent reduction in fuel costs. On the other hand, not all increases in MPG are equal. A 10 percent increase in MPG does not correspond nicely to a reduction in fuel costs—it depends on where you’re starting from. Suppose you drive 10,000 miles a year and a gallon of gas is $2. Going from 10 MPG to 11 MPG is worth $180, while going from 30 MPG to 33 MPG is only worth $60.”