Those cargo carriers you can mount atop your car or SUV may allow you to bring an extra tent for the snoring camper, but they can also significantly cut your gas mileage.
The Department of Energy's website for fuel-economy information recently found that hauling cargo on your roof increases wind resistance and lowers fuel economy by as much as 25 percent if you're driving 65 to 75 miles per hour. You'll get much better mileage with a rear-mount cargo box, which typically lowers fuel economy by just 1 to 5 percent.
