## Scrapping 'mpg' could boost sales of greener cars

2019-03-02 01:11:00

By Catherine Brahic Video: Watch the researchers explain how swapping miles per gallon for gallons per 100 miles could help people make more informed choices over vehicle efficiency What sounds like an arithmetic brain teaser could in fact hold the key to reducing the amount of gas consumed by Americans – and by extension their CO2 emissions. Richard Larrick and Jack Soll of Duke University in the US say that a simple switch from expressing a car’s fuel efficiency in miles per gallon (mpg) to gallons per 100 miles makes it much easier for people to assess how much money they could save on fuel. They showed that people think of fuel efficiency as linear. If this were true, doubling the fuel efficiency of a gas guzzling SUV would save the same amount of fuel as doubling the efficiency of a smaller more efficient car. In fact, the relationship is not linear and doubling the efficiency of an SUV saves far more than if you own a small car (see graph, right). For instance, trading in an SUV that travels 10 miles for every gallon of fuel for another that can go 20 miles, saves 5 gallons on a 100 mile journey. However, trading a small 25-mpg car for a 50-mpg car saves only 2 gallons per 100 miles travelled. The team tested 78 participants in a focus group. These were asked to imagine they were in charge of two municipal fleets of 100 cars. The first was composed of a hundred 15-mpg SUVs; the second was a hundred 34-mpg sedans. They were then asked to reduce their carbon emissions by replacing either the 15-mpg SUVs with 19-mpg SUVs or the 34-mpg cars with 44-mpg cars. Half the participants were given additional information: they were told how many gallons each make consumed per 100 miles. Just one quarter of the less informed group made the right decision – to upgrade the SUVs. In contrast, 64% of those who had the gallons-per-100-miles figures got it right. “The big saving comes from getting rid of the most inefficient car and replacing it with a similar model that’s more efficient,” says Larrick. The team believe their findings could help inform policy makers by showing that making small improvements in gas guzzlers is more urgent than improving on hybrid-car technologies which already are very fuel efficient. “I do believe that there should be lots of reasons to push people to the very high end of the efficiency scale,” says Larrick. “There’s no question about the need for greater efficiency, but if we are simply thinking about getting rid of cars and replacing them, the urgent place to do it is at that low end.” The researchers also think consumers could make more informed choices if car magazines and dealerships helped them make the correct calculations and point out that a metric equivalent of miles per gallon is already used in some European countries. Journal reference: Science (DOI: