Cargo hauled on the roofs of light duty vehicles inhibited fuel economy enough to account for 100 million gallons of gasoline consumption in the United States in 2015, according to a recent study.

Alan Meier of the Lawrence Berkeley National Laboratory and Yuche Chen of the National Renewable Energy Laboratory said this study is the first national look at the effect of roof racks on fuel economy. They determined that this vehicle feature was responsible for 0.8 percent of all light duty vehicle fuel consumption in 2015.

According to the Department of Energy, cargo mounted on roof racks lowers fuel economy by increasing wind resistance. This department says a large rooftop cargo box can cut fuel economy by 2 to 8 percent in city driving, 6 to 17 percent during highway driving, and 10 to 25 percent at interstate speeds of 65 to 75 miles per hour. Removing the cargo box when it is not needed is one way to improve your fuel economy.

Meier and Chen published the results of the study in the journal "Energy Policy." Their research also looked at what policy changes or technology upgrades might be available to mitigate the effect on fuel economy.

"I've always been intrigued by energy consumption that was somehow overlooked or ignored because, for example, it wasn't in the test procedure," said Meier. "In this case, the fuel consumption of vehicles with aftermarket accessories isn't captured in the test procedure."

The study considered a variety of roof rack configurations and their effect on fuel economy in differing situations. These factors included whether or not the roof rack was loaded and how the effect of highway driving differed from the effect of city driving.

Meier and Chen concluded that unloaded roof racks with crosspieces perpendicular to the direction of travel were most likely to contribute to poorer fuel economy. More fuel is wasted in this way because drivers typically travel four to eight times as many miles with empty roof racks as they do with loaded roof racks.

The researchers say efforts to blunt the negative effect of roof racks on fuel economy should focus on reducing the number of vehicles driving with empty roof racks. However, they also project that roof racks will be more popular in coming years, with usage increasing by about 200 percent by the year 2040.

"A national perspective is still needed to justify policy actions," Meier and Chen write in the study. "For comparison, the additional fuel consumption caused by roof racks is about six times larger than anticipated fuel savings from fuel cell vehicles and 40 percent of anticipated fuel savings from battery electric vehicles in 2040."

One option would be for automakers to design roof racks with improved aerodynamics to reduce wind resistance, or roof racks that can be removed when not in use. A policy requiring roof racks to be labeled with their effect on fuel consumption could also encourage improvements in this feature.

Meier and Chen said a government policy to minimize unloaded roof racks would be "admittedly extreme." However, they estimate that such a policy would be able to save 1.2 gallons of gasoline over the next 26 years when combined with more efficient roof rack designs.

The researchers used various methods to collect data for their study. These included crowdsourcing, online forums, and nationwide highway video surveys.