The truth about loading your car with a Christmas tree on top.

Christmas Tree on Car
Credit: The Washington Post/Getty Images

Some of our best holiday memories begin at the Christmas Tree farm. After we carefully select our tree, we happily hoist it onto the top of our car and blast our favorite Christmas tunes (Steve Lawrence and Eydie Gormé's "That Holiday Feeling!" perhaps?). Then we swiftly unload the tree in our driveway and get ready for an evening of decking the halls. Heck, sometimes we even stop for a celebratory hot chocolate break on the way home.

But wait, did you know there may be a drawback to tying your Christmas tree to the top of your car? GMC, a division of General Motors LLC, recently set out to investigate. The company ran their new 2018 GMC Terrain car through a special wind tunnel to look at the so-called "drag impact" (attaching items to the car like a tree or holiday wreath) of this common practice.

WATCH: How To Choose a Christmas Tree

The surprising takeaway from their wind tunnel testing was the strain putting a tree on the roof of your car may put on your gas mileage. According to reporting from Reader's Digest, attaching your Christmas tree leads to a 30% degradation to highway fuel economy. "The Christmas tree causes your drag to almost double. So if you're feeling festive this season, decorate your house, don't decorate your car," said Joel Ruschman, an aero performance engineer for General Motors in a company video. You can watch GMC's full video here.

Meanwhile, if you're looking to mitigate these effects, Ruschman tells Reader's Digest to secure your tree as tightly as you can to your car and travel at slower speeds via back roads on your way home since the tree won't influence gas mileage as much when you drive more slowly.

There's another case for your cousin Rick to add for artificial tree argument.