I have the Weathertech flaps and just finished the first half of a 2,200 mile road trip. On the way out to the east coast,(1,100 miles) my overall mileage was 25mpg and that included the Appalachian mountains and a significant headwind for several hours. Before the mountains and wind, I was running at 27mpg. The Ascent was loaded with four people, luggage, and gifts as well as a roof box. I’d say the Weathertech flaps had no effect on fuel economy.