Phoebe Wall Howard

Detroit Free Press

Rudolph is a drag.

Literally.

General Motors engineers, clearly in a holiday mood, tested the effect of various decorations on gas mileage. (Really, they did!)

They found that adding holiday items to a vehicle — such as reindeer antlers, a bow on top or a Christmas tree strapped to the roof — creates more surface for wind resistance. When the engine must work harder to maintain speed, it uses more gas.

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Yep, holiday decor creates aerodynamic drag.

GM found:

•Antlers. Reindeer antlers and Rudolph’s nose create about a 1-mile-per-gallon decrease in highway fuel efficiency and a 3% increase in drag coefficient — commonly understood as air resistance.

•Bow. A roof-mounted bow creates a 3.5-mpg decrease in highway fuel efficiency and a 15% increase in drag coefficient.

•Christmas tree. A tree tied to a roof creates a 30% decrease in highway fuel efficiency and a 70% increase in drag coefficient. Air pushes against a tree with about 90 pounds of force.

•Wreath. A grille-mounted wreath has no impact on aerodynamics but may reduce cooling airflow to the engine.

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"It might be best to let Rudolph lead Santa's sleigh instead of your ride this holiday season," said Joel Ruschman, GM aero performance engineer.

GM put its new GMC Terrain in a wind tunnel and measured air resistance by determining the vehicle’s drag value when the wind was being pushed through the tunnel at 70 miles per hour, GM explained. The holiday data was part of much bigger research project.

“GMC engineers spent hundreds of hours in the wind tunnel shaping the Terrain’s exterior shape for optimal aerodynamics, and anything added to the outside of the vehicle that disturbs the airflow will contribute to a loss of fuel efficiency,” Ruschman said.

Ruschmann said the team first tested the Terrain by itself to determine the vehicle’s baseline drag value. Then, it added each festive item onto the Terrain individually to see how much drag it added. All the extras might look pretty, but they aren't terribly fuel efficient.

The Grinch would be proud.

