They might look cute and festive during the holiday season, but what are they actually doing to cars? As fun—and somewhat cheesy—as Christmassy car decorations might look on the outside of a vehicle, they can actually be pretty bad for a car’s fuel economy. In December 2017, General Motors performed tests on the impact of holiday decorations on cars and found that having fake antlers, bows, Rudolph noses, and especially Christmas trees on the roof, were harmful to vehicles’ aerodynamics.
Considering how much these decorations raise a car’s wind resistance and minimize efficiency, drivers might want to think twice before trying to channel one of Santa’s reindeer on the road. Here’s the truth about the relationship between car antlers and gas guzzling.
Holiday Car Decorations Are a Serious Drag—Literally
The tests performed by GM engineers found that various holiday decorations had a significant effect on a car’s drag coefficient—that is, how much it impacts its aerodynamics. Tests were done on cars with these decorations in a wind tunnel at a speed of 70 mph (about 112 km/h). First a regular GMC Terrain without decorations was tested to evaluate its base value, followed by one with holiday decorations, to measure the difference in air resistance. The results that came back weren’t exactly encouraging.
For example, mounting a bow on the car’s roof reduced fuel economy by 3.5 mpg and increased its drag by 15 per cent. Rudolph noses and reindeer antlers weren’t kind to gas mileage, either, as they both increased drag by 3 per cent and decreased fuel economy by 1 mpg. However, mechanic school students may or may not be surprised to learn that the biggest culprit is when Christmas trees are tied onto the roof of cars, as that can reduce mileage by 30 per cent and run up its drag coefficient by a whopping 70 per cent.
Mechanic School Students Should Keep These Other Details in Mind
While these numbers—the result of hundreds of hours spent by GM engineers in a wind tunnel measuring its aerodynamics—are enough to dissuade anyone from decorating their car like a Christmas tree, the studies did find that wreaths located on a car’s grille had no impact on a car’s fuel economy. The only situation where it would be a detriment to its gas mileage is if there’s so much greenery that it impedes the flow of cooling air to the engine.
However, students learning how to become a mechanic should know that since these decorations make a larger surface for wind resistance on the vehicle, more gas is used because the engine needs to put in more effort to maintain the car’s speed. Furthermore, trees mounted on top of the car take about 90 pounds of force from the air pushing against it, making it even worse for the vehicle’s fuel economy. In other words, this is definitely more fact than myth.
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