Things are not always what they appear at first glance. A case in point is the 2014 GMC Sierra and Chevrolet Silverado. As a recent photo taken by our own Allyson Harwood shows, the new pickups are visually much more massive than their predecessors. But they are also considerably sleeker. General Motors' new full-size trucks underwent more aerodynamic testing than any truck to date, and the trucks' efficient aerodynamics are one of the key reasons the trucks are able to claim best-in-class V-8 fuel economy, still equipped with six-speed automatic transmissions, just as some of its competitors are starting to use eight-speeds.
GM says one of the most common misconceptions about truck aerodynamics is that driving with the tailgate down or removed gives you better fuel economy than with the tailgate up. Driving with the tailgate closed actually results in airflow "pushing" on the back of the cab, a desirable airflow characteristic. Tonneau covers increase aerodynamic efficiency slightly over an uncovered bed, but somewhat counter-intuitively, soft tonneau covers are more effective, stretching and conforming to the airflow. Also, tubular side steps actually aid airflow around the truck, although flush-mounted running boards (presumably the power-retracting models) are the most efficient.
Cargo nets and bug deflectors were singled out as some of the least beneficial add-on accessories in terms of aerodynamic efficacy, with the cargo nets being described as "dragging a fishing net through water."
Source: General Motors