In a recent “Policy Forum” article in Science, a business professor from Duke argued that we should start reporting gallons per mile instead of MPG. The reason is that fuel used is a linear function of GPM, but an inverse function of MPG. Since humans think best in linear relationships, GPM is the most straight-forward measure to report fuel efficiency.
The real problem with MPG is that the same change in the MPG correspond to a huge change in fuel used at the low MPG end, and almost no change if a car already has a high MPG rating. Going from 20-25 MPG saves more gas that going from 35-50 MPG; going from 12-14 MPG saves more than either. This isn’t intuitive, and you really need to calculate the savings per mile in order to make a rational decision.
The take-away message is that we can’t do calculations like (1/a – 1/b) in our heads.