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5 tips for better gas mileage on summer road trips

If you're planning your summer vacation, you'll want to hold down what you spend for gas so you can spend more on your trip's fun parts. Avoiding common mileage-killing mistakes in how you pack and how you drive can boost your MPG by one-third or more, according to new data from scientists at the U.S. Department of Energy's Oak Ridge National Laboratory.

They measured the penalties to gas mileage of rooftop cargo boxes that make a vehicle less aerodynamic as well as the effects of driving at high speeds during road trips -- among other issues.

John Thomas, a co-author of the study presented last month at a meeting of the Society of Automotive Engineers, summed up the major findings for nonengineers. "Certainly, suitcases strapped your car's roof and trying to keep up with a speeding Ferrari will adversely affect your gas mileage."

For the tests, the national lab's engineers used two 2009 models -- a Ford Explorer SUV and a Toyota Corolla compact. They measured the effects of various changes on both vehicles using laboratory machines similar to those the Environmental Protection Agency uses in estimating MPG on new cars.

Here are some major mistakes to avoid this summer taken from the paper's findings and from the government website

  • Don't use a rooftop cargo box. Adding one to the top of the SUV cut gas mileage when traveling 60 mph from 24.9 to 22.9 -- or 8 percent. The effect in the study was even more pronounced on the Corolla. Its MPG dropped from 42.5 to to 33 with a cargo box -- or 22 percent. With the SUV, one solution is to use a cargo tray mounted behind the vehicle near the rear bumper. Its effects on MPG were negligible.
  • Don't drive aggressively. Speeding up then braking sharply over and over can cut highway mileage by 33 percent, according to In addition to burning more gas, such driving makes you more likely to get into an accident.
  • Don't drive too fast. The government site cautions against too much speed, noting that every five mph you drive above 50 mph is the equivalent of paying an additional 24 cents a gallon for gas. Chances are you don't want to chug along at 50 mph on the Interstate. But the Oak Ridge study showed that driving 80 mph cut the Corolla's mileage by 46 percent below the MPG at its optimal speed of 40 mph. The 80 mph penalty for the Explorer was 40 percent.
  • Don't drive with the windows down. Many drivers believe they're improving their gas mileage by driving with the windows down instead of turning on the air conditioning. But the Oak Ridge tests showed that actually increased aerodynamic drag and cut MPG by up to 8.5 percent for the Corolla and 4 percent for the Explorer. When driving 60 mph and using the air conditioning at half-strength, "there's likely to be no fuel economy penalty," the report says.
  • Don't drive on underinflated tires. The Oak Ridge tests showed that tires allowed to get down to 50 percent of the recommended inflation level cut gas mileage by up to 10 percent. More important, such severely underinflated tires increase the danger of accidents, especially rollovers.

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