Those Rooftop Reindeer Antlers Slay Your Car's Fuel Economy

CARS.COM — Showing your holiday cheer by dressing up your car like Santa's sleigh will cost you, according to engineering elves at GM. Turns out, those cute reindeer antlers, Rudolph noses and big red bows can be a real drag on fuel economy by increasing a vehicle's wind resistance — and don't even ask what a Christmas tree on the roof does to your vehicle's drag coefficient, the measure of how aerodynamically "slippery" it is.

Related: How to Transport Your Christmas Tree Safely by Car

GM engineers, being engineers, precisely tested this festive holiday kitsch in a wind tunnel and calculated what it does to their vehicles' carefully crafted aerodynamics. They found that:

• Reindeer antlers and Rudolph's shiny schnoz increase the drag coefficient by 3 percent and cut highway gas mileage by 1 mpg.
• A bow on the roof raises drag by 15 percent and cuts fuel economy by 3.5 mpg.
• Perhaps most Grinch-like: a Christmas tree tied to a roof, increasing drag by 70 percent and cutting your mileage by 30 percent. (Safety warning: The tree is under about 90 pounds of force from the air, so be sure you've got it up there securely.)

"If you're feeling festive this holiday season, decorate your house, don't decorate your car," said Joel Ruschman, GM aero performance engineer. The engineers first tested a naked GMC Terrain in the wind tunnel for a base value. Then they added holiday decor to measure the effects of the holiday spirit's physical manifestation at 70 mph.

The results aren't entirely humbuggy: They found that a wreath on the grille doesn't cost you (unless the greenery is so extravagant that it blocks cooling air for the engine).

You can check out the GM Grinches at work testing the Terrain by watching the video below.

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