[JRTJH]

Ford (and I'm sure other manufacturers as well) have done an excellent job of "smoothing the airflow" on today's vehicles. They have gone so far as to redesign the windshield wipers and their location, park position and function. Hoods, windshields, rooftops, truck beds, tailgates, tailgate "dams" have all been designed with airflow function as a primary concern. Even small, seemingly insignificant things like the curve of the rear of the cab have been designed to not disrupt airflow.

Hanging a 5' wide "wing" on top of today's trucks is significantly disruptive to the airflow over the vehicle. When engineers change the slope of the windshield 1 degree and reposition the wipers 1/4" up or down, to "fine tune" the airflow, I find it hard to believe that much "square bulk" hung to the roof can be beneficial or an improvement. For me, it seems more "snake-oil" than engineered science... YMMV