Car manufacturers rigorously test new vehicles in wind tunnels to trim away tiny bits of cladding or design elements that negatively affect the car’s aerodynamics and, most importantly, its performance. Slight alterations to the exterior of the car can have a significant impact on fuel economy.
When Buick launched its new, , billed as an adventure vehicle, engineers wanted to shed some light on how attaching various outdoor gear to the roof of your vehicle would impact your gas mileage.
MensHealth.com was invited to join Buick during wind tunnel testing at a Lockheed Martin low-speed wind tunnel facility outside Atlanta, Georgia. Over the course of nearly 11 hours spent, we tested different configurations of outdoor gear strapped to the top of the new TourX, ranging from plain roof rails and mountain bikes and a paddle board.
Although it’s not the only important factor, the main metric observed when wind tunnel testing is the coefficient of drag. The lower this value, the easier a car slips through the air. When you begin attaching things to the top of an otherwise very aerodynamic vehicle, the coefficient of drag increases and everything you think you know about your car’s gas mileage goes out the window. Just how much it increases, and how much it impacts your car’s fuel economy, is exactly what Buick hoped to ascertain.
Off the showroom floor, the Regal TourX gets 21 miles per gallon in the city and 29 mpg highway for a combined fuel economy of 24 mpg.
The control test—the TourX without roof rails installed—revealed that adding roof rails alone decreased the car’s fuel economy by about seven percent, or a negative -2 mpg change. Beyond that, the two items tested with the least impact on gas mileage were the paddle board and Thule luggage carrier, both of which caused about a -19 percent decrease in fuel economy, or -5.5 mpg.
The gear with the most impact were the dual mountain bikes strapped to the roof, which impacted the fuel economy by -31 percent or -9 mpg. A close second was the kayak with -28 percent, or a change of -8 mpg.
To add a little fun to the mix, we also tested a large, (because why not, right?) and strapped it down to a Thule basket. It caused a whopping 34 percent drop in fuel economy, or -10 mpg. Of course, no one in their right mind would drive around with an oversized pool float fastened to the roof of their car while it’s inflated.
But the point stands: anything you strap to the top of a car is going to affect your gas mileage. The larger in surface area the item, the larger the impact on the coefficient of drag. And the more aerodynamic the items you strap to your car are, the less they should affect your fuel economy.
Using a roof rack and toting bulky equipment on top of your car isn’t the only thing affecting your gas mileage, however. Other modifications you make to your vehicle — particularly those to the appearance or outside of the car — can affect gas mileage, as well. For example, lifting a car, even an inch, can negatively impact the fuel economy (while lowering can increase it). Using narrower tires can improve your gas mileage.
While you clearly shouldn’t stop hauling your kayaks or mountain bikes on top of your car, maybe you should take the time to take off the roof rack if you’re not going to use it for a while. Or deflate your pool floats before driving home.