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Tool helps overcome miles-per-gallon illusion

A new tool helps motorists evaluate the fuel efficiency of their vehicle in terms that more accurately reflect the cost of driving than miles-per-gallon (MPG).

The Gallons Per Mile Calculator, put together by Duke University professors Richard P. Larrick and Jack B. Soll, enables users to calculate fuel consumption in gallons per mile. The measure is important because it helps drivers overcome the “numeric illusion” associated with the conventional MPG metric.

Larrick and Soll highlighted this illusion in a paper published in the journal Science earlier this year. Conducting three experiments to test whether people reason correctly about gas mileage, the professors 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 wrote.

“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.”

Larrick tells that the new tool aims to do just that.

“These calculators let people see the actual gas consumption (and savings) of different cars,” he said. “One calculator can be used for a choice of MPG, distance, and gas price. Two other calculators allow consumers to examine new 2009 cars and evaluate them based on GPM.”

“These calculations are critical to helping people see their true gas consumption (and carbon emissions) and not be tricked by MPG.”

Larrick says he hopes either the EPA or Consumer Reports adopt GPM to give consumers better information for making smart purchasing decisions.

Gallons Per Mile Calculator