There’s a Hidden Downside of Driving with a Christmas Tree on Your Car

It affects your gas mileage a lot more than you might have guessed.

Tis the season… to head to your favorite Christmas tree farm, chop down the best-looking tree, and strap it to the roof of your car to get it home so you can start trimming it. But have you ever thought about how much having a (probably pretty full) tree on top of your car affects its gas mileage? Well, GMC did a little experiment to see just how much your highway fuel economy is compromised when you’re bringing home this year’s Christmas tree and set up their new GMC Terrain in an aerodynamics lab to test it out.

(These are secrets your Christmas tree wishes you knew.)

The car was put in a wind tunnel and air resistance was measured by determining the vehicle’s drag value when the wind was being pushed through the tunnel at 70 miles per hour (which is equivalent to the car traveling at highway speeds).

Courtesy of GMC

“By adding something to the vehicle, you are giving the air more surface to push against as you drive down the road. The harder the air pushes against the vehicle, the harder the engine has to work to maintain speed, which uses more gas,” said Joel Ruschman, Aero Performance Engineer from General Motors. “A Christmas tree on your roof is a fairly large object with lots of surface area due to its needles and branches, so this creates a lot of aerodynamic drag.”

The final results? Having a Christmas tree on the roof of your car results in a 30 percent degradation to highway fuel economy. That’s a lot! They also tested various decorations that people add to their vehicle during the holidays. Having reindeer antlers and Rudolph’s nose tied to your car results in about a 1 percent decrease in highway fuel economy and having a bow of your roof results in about a 3.5 percent decrease. Having a wreath mounted on the grille of your car has no impact on gas mileage but it does reduce cooling airflow to the engine.

Ruschman suggests tying down your tree as tightly as possible and traveling on back roads if you can—having a tree on top of the car won’t affect gas mileage when you’re moving at lower speeds. And, as a plus, you’re less likely to have a Christmas tree disaster on the highway. (Driving a little slower can actually save you money on gas, so you should give it a try.)

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