Anything you attach to the top of your car, including the roof rack, is going to affect your gas mileage. The larger the item, the greater the impact.
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.