Fake antlers, bows, and a Christmas tree tied to the roof could up your car’s fuel bill by 30 percent, according to recent wind tunnel testing by General Motors.

Using a GMC Terrain as its lab rat, the automaker found out that all those holiday car decorations people stick on their car are not only visually but literally a drag, and broke out the aerodynamic inefficiency of several common pieces.

Reindeer antlers and a Rudolph-style nose on the hood upped air resistance by three percent, for example, driving your gasoline consumption up by a full mile-per-gallon.

A bow on the roof caused a 15 percent increase in drag coefficient, docking drivers 3.5 miles per gallon; and a Christmas tree tied to the roof represented 90 lbs of drag, which would bump up your bill at the pump by a full 30 percent.

Your best bet might be a grille-mounted wreath, which didn’t noticeably affect the vehicle’s aerodynamics (though it might hinder your engine’s cooling).

“It might be best to let Rudolph lead Santa’s sleigh instead of your ride this holiday season,” concluded Joel Ruschman, GM aero performance engineer.

The testing was conducted by the automaker inside a wind tunnel, at a simulated 70 mph (110 km/h).