Gallons Per Mile

We are used to quoting miles per gallon (MPG) to measure a car’s energy efficiency. And we all know that a 9 mile-per-gallon suburban assault vehicle contributes far more to global warming than a new gasoline-electric hybrid that gets 50 miles to the gallon.

But which of these theoretical situations does more to curb global warming: switching from a 18 MPG SUV to a 28 MPG model; or trading in a 42 MPG hybrid for a 48 MPG hybrid upgrade?

Most of us would answer that the latter is correct, and we’d be wrong.

In the 20 June 2008 issue of the journal, Science, authors Richard P. Larrick and Jack B. Soll of Duke University discuss the results of a study they conducted on perception of fuel efficiency. They concluded that if we were to view automobile efficiency in gallons per mile (GPM) instead of MPG we’d have a much clearer view of how much gasoline a particular car uses.

What trips us up, it turns out, is that we assume that gasoline consumption decreases linearly as MPG increases. In reality the relationship is a curve. The least efficient engines use the most gasoline per hundred miles. As MPG increases, the gasoline used per hundred miles curves downwards so that the change in GPM from 15 to 25 MPG is greater than the change in GPM from 40 to 50 MPG.

In the theoretical examples above, improving MPG from 18 to 28 reduces gasoline consumption by 1.984 gallons over 100 miles, whereas going from 42 to 48 MPG saves only 0.298 gallons over 100 miles. The consumption reduction is 6.66 times more in the first example than in the second.

To make the differences clearer, let’s compare two pairs of cars. Within each pair there is only a 5 MPG difference, but in the second pair are higher MPG cars:

An old pick-up that gets 10 MPG uses 10 gallons per 100 miles. A 15 MPG SUV uses 6.67 gallons over the same distance. The difference between the two is 3.33 gallons per hundred miles.

A reasonably efficient sedan that gets 30 MPG uses 3.33 gallons per 100 miles. A more efficient sedan that gets 35 MPG gets 2.86 gallons per 100 miles. The savings in gas consumption here is 0.47 gallons per hundred miles.

So, while the more efficient cars consume far less gasoline over 100 miles, there is a greater benefit in removing the least efficient cars from the road and replacing them with moderately efficient ones than there is in trading an efficient car in for a super-efficient one.

Ideally, of course, we’d all be driving cars that would use 2 gallons or less per 100 miles. That’s 50 MPG, by the way.