Forget MPG - Let's Move to GPM

A simple proposal for changing how we measure fuel-efficiency that could have a profound effect on the cars Americans buy and how much fuel they consume:

There's already a lot of calculation needed to figure out how efficient a vehicle is. Here in the U.S., the government tries to help by publishing official MPG numbers of the various models offered for sale. But these numbers just muddy things up, say two management professors - Richard Larrick and Jack Soll - over at Duke University. Thinking in terms of miles per gallon doesn't give people a good understanding of a vehicle's real efficiency when compared to other vehicles.

In Europe, for example, mileage figures are given in liters per kilometers. There are online calculators we can use to figure out what the MPG equivalents are, but the Duke profs think we should be moving to a gallons per mile model here in the U.S. That way, people might begin to see that trading a 14mpg SUV for a 21mpg hybrid version, for example, saves more fuel than trading in a 35mpg sedan for a 50mpg Prius. Right now, very few consumers realize this when they're out debating which car to buy. Sure, a Prius burns less fuel than a SUV hybrid over the same distance, no question, but there's something to be said for the seemingly mediocre fuel economy improvements made in the low-mpg segments.

Listen to this 3-minute NPR interview with Richard Larrick.

Here are some actual numbers to put this into more concrete terms:

To drive the 11,000 mile journey across the US my dad and I took in 2005, here's how much gas you would consume if you drove a 15 MPG vehicle, a 30 MPG vehicle, or one of those new cars that will get 235 MPG:

Gas Mileage	Fuel Required
15 MPG	11,000 miles/15 MPG = 733 gallons
30 MPG	11,000 miles/30 MPG = 367 gallons
235 MPG	11,000 miles/235 MPG = 47 gallons

According to these calculations, you will get a bigger fuel savings (733 - 367 = 366 gallons) changing from a 15 MPG vehicle to a 30 MPG vehicle than you would going from a 30 MPG vehicle to a 235 MPG super-car (367 - 47 = 320 gallons).

Counter-intuitive, but true.

P.S. -- Below is a graph I made showing the inverse relationship of miles per gallon (MPG) and gallons per mile (GPM) measurements of fuel efficiency. The inverse relationship is non-linear, helping explain why it is a non-intuitive relationship for most. (People have a much better natural intuition for understanding linear relationships than non-linear.)



(HT Glenn Reynolds. Fuel gague photo by Laffy4k.)