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Miles-per-gallon misrepresents gains in fuel efficiency from scrapping worst gas-guzzlers

MPG misrepresents gains in fuel efficiency from scrapping worst gas-guzzlers

MPG misrepresents gains in fuel efficiency from scrapping worst gas-guzzlers
June 20, 2008

The use of miles-per-gallon instead of gallons-per-distance to measure fuel-efficiency may be clouding Americans’ judgement when it comes to choosing whether to take the worst gas-guzzling vehicles off the road, argues a new paper published in the journal Science.

Conducting three experiments to test whether people reason correctly about gas mileage, Richard P.
Larrick and Jack B. Soll of Duke University found students “consistently undervalued small gains in fuel efficiency for the most inefficient cars.”

For example most students responded incorrectly to the question whether the savings of going from a 12-MPG car to a 14-MPG car were greater than replacing 28-MPG auto with a 40-MPG model. The 2-MPG gain from upgrading from a 12-MPG to 14-MPG translates to a savings of roughly 120-gallons over the course of 10,000 miles. By comparison, going from 28-MPG to 40-MPG saves 95 gallons over that distance.

“The environment would benefit most if all consumers purchased highly efficient cars that get 40
MPG, not 14, and incentives should be tied to achieving such efficiency. An implicit premise in the example, however, is that an improvement from 12 to 14 MPG is negligible. However, the 2 MPG improvement is actually a significant one in terms of reduction in gas consumption,” the authors write.

The research shows that most people fail to see that the amount of gas consumed by an automobile decreases as a curvilinear function — rather than a linear function — of a car’s MPG. The authors attribute this to “numeric illusion” for U.S. consumers who see fuel efficiency expressed as miles per gallon, rather than gallons consumed per unit of distance.

“Removing the most inefficient vehicles is where policy and popular opinion should be focused,” they write. “Representing fuel efficiency in terms of amount of gas consumed for a given distance—which is the common representation outside of the United States (e.g., liters per 100 kilometers)—would make the benefits of greater fuel efficiency more transparent.”

Larrick and Soll say fuel efficiency is just one example of the ned for a “common metric” for comparing carbon emissions across a range of daily actions.

“Arming consumers with information about the relative greenhouse gas emissions of various activities expressed in a common metric can allow concerned consumers to make beneficial trade-offs in their daily decisions.”

R.P. Larrick and J.B. Soll (2008). The MPG Illusion in Judging Fuel Efficiency. Science 20 June 2008

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