Trading in your least fuel-efficient car for a marginally more economical one might ultimately benefit your wallet more than upgrading the most fuel-efficient car in your driveway.  That may not be obvious from the mpg ratings of the vehicles.  But, new research suggests, it becomes clear if you reverse the way you think about fuel efficiency.

According to Reuters, a report from Duke University researchers, published in the journal Science this week, suggests that consumers have an easier time figuring out how fuel efficient a vehicle is if fuel economy is expressed in gallons per mile, instead of miles per gallon.  Lead researcher Richard Larrick tells Reuters, "People think improvements in miles per gallon are all the same, where a 5 gallon per mile improvement would yield the same gas savings in a car that gets 10 miles per gallon or 20 miles per gallon." 

In fact, Larrick tells the U.K.'s Guardian, "replacing a car that does 10mpg with one that appears only slightly more efficient at 11mpg saves as much fuel as upgrading from a 33mpg car to a 50mpg car."  

So trading in your Jeep Grand Cherokee for a marginally more fuel-efficient Dodge Journey could ultimately do more good for your bank account than downsizing from a V6 Accord to a smaller, four-cylinder Civic. 

Autoblog points out that "Europeans already express their fuel usage in liters/100 km, which makes sense seeing that they've been paying $5 a gallon for much longer than we have."

Research the most fuel-efficient options in every class with U.S. News' car rankings and reviews.