The summer driving season is drawing to a close and the end-of-year car sales season is nigh. Naturally, then, I am thinking about fuel efficiency. Specifically, I am questioning whether miles per gallon (MPG) is the right way to think about cars and gas.

To explain, let’s start with an arithmetic quiz. In terms of saving additional gallons, which is more effective: turning in a vehicle that gets 12 MPG for one that gets 15 MPG or one that goes from 33 MPG to 50? At first blush, the answer is obvious; the latter is a 50 percent increase in mileage, so that must save more gas than the former, which is only a 25 percent increase. QED. And wrong, by a lot (see chart).

To understand why, let’s consider a different question: How many gallons does it take to go a certain distance? A 12 MPG car swallows 833 gallons to go 10,000 miles (distance divided by miles-per-gallon). An 15 MPG car takes 666 gallons. The savings, then, is 167 gallons. That is two-thirds more than the savings of going from 33 MPG (300 gallons consumed) to 50 MPG (200 gallons) over the same distance.

Duke University economists Richard Larrick and Jack Stoll might have been the first to explore this concept. In 2008, they wrote an article for Science magazine called “The MPG illusion”; they explain it further in this video. They also note that MPG isn’t particularly useful in terms of evaluating the dollars-and-sense of driving decisions, which is what matters most to most people.

Say your summer holidays will require you to drive 1,000 miles to visit both sets of grandparents and one amusement park. Should you take the roomy car with lots of room for toys? Or the smaller one? One important factor—besides the temperament of the children involved—is surely cost. At $2.72 per gallon (the US average), it takes $181.15 to drive a 15 MPG vehicle on this vacation and $90.57 for a 30 MPG one. In California, where a gallon costs $3.59, the difference is $119.54 for the more efficient car, compared to $239.09 for the guzzler. As an economist would put it, those are non-trivial differences; GPM is a much better way of calculating the financial cost of driving. (Click here for a handy calculator.) Environmentally, the math is also easy. Taking the smaller car saves 33 gallons—that translates into about 660 pounds fewer carbon emissions.

It’s unlikely that Americans are going to get MPGs out of their heads, any more than we have adopted the far more sensible metric system. The status quo may be inferior, but it is implanted deep in our brains. And of course it’s important to remember that a more fuel-efficient car is much, much greener. A 50 MPG car uses 633 fewer gallons of gas than an 12 MPG one over 10,000 miles[i] and emits fewer than a third of carbon emissions; also, fewer resources were consumed to make it in the first place (gas guzzlers tend to be big).

That said, the GPM idea is worth keeping in mind. The Environmental Protection Agency seems to agree; beginning with 2013 models, it began to require something like GPM on new-car labels.

Explicitly citing the “MPG illusion,” the EPA notes that MPG “can be potentially misleading when consumers compare fuel economy improvements, particularly when they use it in place of fuel costs.” The EPA’s new labels also include an estimated dollar costs/savings over five years (compared to the average); that is an effort to put both cost and efficiency into perspective. It’s progress, but the labels are difficult to decipher and the GPM figure is not prominent. Also, the fuel-economy database does not include GPM. MPG has its own web-page; GPM is an orphan.

More consequentially, there is a case to consider that in terms of policy, it’s more important to raise the performance of gas-guzzlers even a little than to improve that of already-efficient vehicles. The latter is sexier because everyone likes to see big numbers. But the fact is that while both are good, one does more good, faster, at less cost.

Chart credit: www.mpgillusion.com

[i] The average American drives 13,476 miles a year.