General Motors has done a number of experiments with popular Christmas decorations and ornaments on their vehicles and found that they are bad for cars. They mess with the aerodynamics of vehicles to a measurable and significant level and GM suggests that drivers not use them this holiday season.

For those who love decorating their homes in Christmas cheer, it is recommended that ornaments not find their way to vehicles. Adding certain items to cars increases drag on them, reducing their efficiency, a study has found. GM, one of the world's largest automobile manufacturers, put a number of Christmas decorations including antlers, Rudolph noses, and wreaths on their cars and placed them in wind tunnels.

"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.

The findings of the study as reported by the Detroit Free Press:

Reindeer antlers and Rudolph noses on the hood of a car can reduce highway mileage by one mpg. They also increase the drag coefficient – air resistance around the car– by 3%.

A bow mounted on the roof of cars leads to a 3.5 mpg decrease in highway fuel efficiency. Drag is increased by 15%.

A small Christmas tree mounted to the roof of a car can cause a 30% reduction in highway fuel efficiency and a whopping 70% increase in drag on the car. Air will push against roof-mounted trees with over 90 pounds of force.

Grille-mounted wreaths do not affect the aerodynamics of the car, but will reduce the amount of cool air that enters the radiator and other systems of the car.

The vehicle used for this test, according to GM, was its all-new GMC Terrain SUV. It was placed in a wind tunnel and air resistance was tested at a speed of 70 mph. The results that were gleaned off the study came as part of a much larger 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.

While the study made use of only type of vehicle and specific decorations, it is not clear if the same results hold good for other cars from other car makers as well. Either way, it can be safely assumed that adding large ornaments to cars is simply not a good idea.