GMC’s aerodynamics engineer Joel Ruschman put a GMC Terrain with typical Christmas decorations through the Lockheed Martin wind tunnel testing in Marietta Georgia. The biggest offender of fuel efficiency is a Christmas tree tied to the roof.
He found that some holiday decorations can affect performance cause drag on a vehicle including a new 2018 GMC Terrain. The 2018 GMC Terrain decreased drag a lot and improved fuel economy however savings could be lost with air resistant decorations.
Antlers & Reindeer Nose – Reindeer antlers on the roof and Rudolph’s nose on the grill cause a 1-mile-per-gallon decrease in highway fuel efficiency and a 3% increase in drag coefficient (air resistance).
Bow- A fluffy bow on the roof (without Santa) causes a 3.5-mpg decrease in highway fuel efficiency and a 15% increase in drag coefficient.
A Christmas Tied on Roof causes a 30% decrease in highway fuel efficiency and a 70% increase in drag coefficient. Air pushes against a tree with about 90 pounds of force.
A Full Frontal Wreath on the Grille is not going to hurt you at all but may reduce cooling airflow to the engine.
Some things Ruschman didn’t note is what happens when the decorations or tree fall off the car and if the vehicle has automatic Santa braking. “I Brake for Reindeer” license plate optional.