What would happen if the U.S. started using gallons per mile instead of miles per gallon as a measurement of vehicle fuel efficiency? Two management professors at Duke University say the switch could help car buyers get a better read on the real cost of an inefficient car.

According to the researchers, whose study appears in the journal Science, most people in the U.S. think an improvement of 5 mpg yields the same savings (in terms of fuel and dollars at the pump) regardless of the starting point--20 mpg to 25 mpg, for example, or 30 mpg to 35 mpg. The study's lead author told Reuters that this is "a math illusion."

In reality, the amount of fuel you need to drive say, 100 miles, does not decrease evenly as the miles per gallon improve. That means a 10 mpg improvement can deliver more savings than a 15 mpg one--depending on where you start.Boston.com breaks it down:

Swapping a 34 MPG car with a 50 MPG car would save about 94 gallons for every 10,000 miles driven. But switching a 18 MPG car with a 28 MPG one would save 198 gallons.

It turns out small gains in the biggest gas guzzlers might be helping the environment more than people think. Of course, the environment would benefit the most if everyone was driving Priuses. But the research does indicate US drivers may be underestimating the real value of getting the worst gas-guzzling cars off the road.

Want to crunch the numbers for your own car (or for your neighbors' vehicles, if you're into eco-one-upmanship like Sandra Tsing Loh)? Study author Richard Larrick has put together a simple calculator on his blog. If you check it out, we'd love to hear about your results.