When thinking about saving gas, it's useful to remember that not all mpg's are created equal.

Among very fuel-efficient cars, the incremental savings for every extra mpg is small. However, among very inefficient cars, even a 1 mpg savings can be significant.

Researchers at Duke University Fuqua School of Business recently released a study showing that consumers often make poor decisions when it comes to fuel savings.

For example, consider a family that owns a Chevrolet Tahoe as a family vehicle and a Toyota Camry as a commuter car. The Tahoe gets 14 mpg and the Camry four-cylinder gets 24 mpg, according to Consumer Reports testing. (Mileage figures used to illustrate the mpg mathematics are based on late-model cars.) Assume each vehicle is driven the national average of 12,000 miles per year. In an effort to save fuel, the family might be tempted to buy a high-mileage hybrid. To maintain the convenience of the Tahoe, they consider trading in the Camry for a Honda Civic Hybrid that gets 37 mpg—a 13 mpg improvement. This transaction would save them 176 gallons of fuel a year. At $4 a gallon, that would be a savings of about $700 in gas alone.

But the savings could be even greater if they considered replacing the Tahoe with a Toyota Highlander that still seats seven and gets 18 mpg. While that’s only a 4 mpg improvement, the total savings in replacing the gas-guzzling Tahoe would be 190 gallons a year, or $762 in fuel.

While the Civic Hybrid might yield better bragging rights, differences of a few miles per gallon are amplified with cars that get poor fuel economy, while cars that get good fuel economy face diminishing returns for each additional mile per gallon. Of course, when considering downsizing, it is important to look at the complete owner cost situation, as trading in too soon (say, mid-way through a loan term) may not be the best deal in the long run. (Read “When to downsize your car.”)

To help consumers make better comparisons, the Duke researchers propose comparing fuel consumption—gallons per mile—rather than fuel economy in miles per gallon. This would help consumers make more direct comparisons between fuel costs, emissions, and a car’s contribution to global warming or global oil consumption.

In addition to our fuel economy measurements, Consumer Reports also translates that into annual fuel consumption and costs in our model pages, available through pulldown menus on the major Cars pages and through the New Car Selector.

The chart embedded in this post illustrates the numbers, based on rounding the current national gasoline price average of $4.06. To look at this in a different way, try the university's "The mpg illusion" interactive quiz.

This Duke study shows the importance of knowing how numbers translate into real dollars and supports the notion that it is key to factor the total owner costs when considering buying a new vehicle, especially if pain at the pump is the prime motivator. Ultimately, every gallon saved helps not only your wallet, but also aids national conservation and benefits the environment. Just make sure you understand the full picture before you buy that next car.

—Eric Evarts

For more information on saving fuel see our reports on how to get the best gas mileage and where to find the cheapest gas.