Believe it or not, folks, but this THULE arrangement is horrible for best mileage. Disregarding the fact a bike is completely in the airstream, we will ignore it and focus on the box. Backward mounting would be best and here’s why.
Simply put, the turbulent air flow is what adds drag and kills mileage. It comes from the separation of air from the surface along its travel. Yeah yeah, we know this. We study race cars and we see drafting and slipstreaming all day long in NASCAR (if you care to watch that) so why do the aero-box users persist in mounting the sleek looking “aerobox” in the promoted blunt end back direction?
The ability to keep the airflow smooth as it leaves the rear of the body makes for less turbulence and less drag. Airplanes rarely have pointy noses unless they’re splitting the sound barrier. The most efficient Boeing/Airbus products have a rounded nose and knife edge or pointy tails. Spin it around and you have a blunt front (actually good) and a smooth teardrop shape for the air to flow with, leaving a smoother less turbulent/low drag ending behind.
Especially in the truck case, backward would be perfect as the airflow off the roof wouldn’t even touch the blunt end very much at all and just conform to the box.
Better mileage for you, and if you’re the judgy sort, you can now look at everyone else doing the top picture idea and eye-dagger them while smiling, knowing, as a superior motorist in a sea of depressing Luddites.
....what a terrible way to live, all judgy like.