Car designers tune their shapes in wind tunnels to allow their vehicles to slip through the air with the greatest of ease. That helps gas mileage. To check the effects of screwing up their efforts, Car and Driver tested mileage with a large, square-cornered rooftop carrier in place, noting a net loss of 6 mpg. Contemporary ski racks and gear boxes are more streamlined and carefully contoured to minimize this penalty. That said, it’s a good idea to remove roof racks that aren’t being used and only install them when you need them.
It’s also a good idea to repair flapping fenders, dragging mufflers and damaged trim jutting into the air stream. Over thousands of miles, seemingly small effects add up to gallons of wasted fuel, especially at highway speeds. To see how a significant deterioration of the aerodynamic drag coefficient impacts mileage, Car and Driver propped open the front of the hood on a test car one foot. That sapped mileage by 7 percent.
