Even Modest Increases in MPG Can Equal Big Gas Savings

Brian Handwerk
for National Geographic News
July 2, 2008
Sky-high gas prices this summer could have many drivers in the United States looking for a change, and most are well aware that buying a vehicle that gets better miles per gallon (mpg) will save them more dollars at the pump.

But not everyone looking for a switch wants to sacrifice passenger and cargo space by switching from a gas-guzzling SUV to a hyper-efficient compact.

What most Americans don't know is that even small boosts in efficiency can often save more fuel overall than much larger mpg gains in already-efficient vehicles, according to a new study.

In the paper, researchers at Duke University in North Carolina suggest that car buyers looking to upgrade their fuel efficiency need to think not in terms of mpg but in gallons per mile (a gallon is equal to about 3.8 liters).

Study co-author Richard P. Larrick said the idea was inspired by his own family's experience.

"We replaced a minivan with a station wagon. But it wasn't [immediately] obvious that if we replaced an 18-mpg vehicle with a 28-mpg [vehicle, that] it would be better than [upgrading the vehicle] from 33 to 50 mpg," he said.

"Once we did the math, we discovered it saved twice the gas, given our driving."

Making a Trade

For the new study, published last week in the journal Science, Larrick and Duke colleague Jack Soll polled consumers about fuel efficiency.

Many respondents didn't realize that a larger increase in the miles a car can drive on a gallon of gas does not always correlate to a greater increase in fuel savings, the authors found.

The researchers suggest that fuel-use comparisons between vehicles are far easier to understand when listed as total consumption per 10,000 miles (16,093 kilometers).

For example, an SUV that gets 10 mpg needs 1,000 gallons of gas to drive 10,000 miles. A 16-mpg station wagon, meanwhile, needs only 625 gallons to go the same distance.

That means trading the SUV for the station wagon would save 375 gallons of gas.

By contrast, trading a 35-mpg sedan for a 50-mpg hybrid would save just 86 gallons of gas.

(Related: "Hybrid Cars Losing Efficiency, Adding Oomph" [August 2005].)

The researchers therefore recommend that Americans should focus on upgrading the worst gas guzzlers first.

"Which car should everyone be driving? They should be driving the 50-mpg car. But which one is more important to replace? It's that 10-mpg car," Larrick said.

"You don't have to replace [the gas-guzzler] with a 50-mpg car to save a lot of gas," he said. "You can replace it with a 20-mpg car and save a lot of gas."

"Crusher Credit"

U.S. Department of Energy researcher David Greene, who writes for, agrees that a change in how fuel efficiency is reported could help consumers make smarter choices.

"People working in this area have known for a long time that this is a problem and a cumbersome way to report fuel-economy numbers," said Greene, who works at Oak Ridge National Laboratory in Tennessee.

"In Canada and in Europe they use liters per a hundred kilometers, which obviously gives a better sense of the rate of fuel consumption," he said.

"Some auto companies, Honda for example, have been very much in favor of a change," he said. But so far U.S. officials seem reluctant to change a familiar system.

Therese Langer, a transportation expert with the nonprofit American Council for an Energy-Efficient Economy (ACEEE), advocates what she dubs a "crusher credit" to boost fuel efficiency among U.S. cars.

The controversial idea would pay drivers to scrap their low-mileage gas guzzlers.

"While we're trying to bring [new vehicle gas mileage] up as quickly as feasible in a cost-effective way, we also want to get some of these gas-guzzling vehicles off the road," she said.

Now is a good time, she said, because drivers of low mpg cars and trucks "may be suffering buyer's remorse with the gas prices they are paying."

But going after gas-guzzlers doesn't necessarily mean scrapping older vehicles.

"[Improvement in] fuel economy was basically flat from the late 1980s until last year," she said. "There's not really a good correlation between the age of the vehicle and how much [gas] it consumes."

Larrick, the study author, stressed that he doesn't want his research to be misconstrued as an invitation to drive anything other than the most fuel-efficient car compatible with a person's needs.

"We're not advocating for people to buy low mpg vehicles," he said. "We are advocating for people to replace the most inefficient cars with more efficient ones."

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