People who pack their cars and drive like Clark Griswold in National Lampoon’s “Vacation” pay a steep penalty when it comes to fuel economy, according to a report by Oak Ridge National Laboratory.

For the study, researchers tested a sport utility vehicle and a compact sedan with various configurations, including underinflated tires, open windows, and rooftop and hitch-mounted cargo. The SUV, a 2009 Ford Explorer with a 4-liter V6 engine, was also tested while towing an enclosed trailer. The researchers tested the vehicles at a variety of speeds with the different configurations. While the findings were not unexpected, they serve as a reminder of how drivers can save money by taking simple measures.

“There is fuel economy information and advice available for vehicle maintenance and carrying loads that is quite good, but very little published data to back it up,” said John Thomas, a co-author of the study and member of ORNL’s Energy and Transportation Science Division. “Certainly, suitcases strapped to your car’s roof and trying to keep up with a speeding Ferrari will adversely affect your gas mileage.”

Among the more notable findings was that using a rooftop cargo box with the SUV decreased fuel economy from 24.9 mpg at 60 mph to 22.9 mpg—a drop of 9 percent. The compact sedan, a 2009 Toyota Corolla with a 1.8 liter four-cylinder engine, also suffered as its fuel economy dipped from 42.5 mpg at 60 mph to 33 mpg, or 22 percent, when hauling the rooftop cargo box. [Read more…]