Holiday decorations arguably are the main reason the dark winter months are filled with so much cheer. They’re also the reason roadways are filled with so many ugly cars.

Festive accessories for cars rarely look great, but if you disagree, GMC has another reason that might dissuade you from turning your vehicle into a rolling Rudolph: money.

GMC recently posted a video, in which it conducted wind tunnel tests with a 2018 Terrain Denali to determine if/how much different decorations impact drag and fuel economy. And the automaker’s findings might surprise you.

Although a 1 mpg drop in fuel mileage from the antlers and red nose doesn’t sound like much, it’s important to remember they have that affect every time you drive. Considering most people who decorate their cars do so for at least a month, it wouldn’t take long for those adornments to eat an entire gallon of fuel.