GM said it analyzed which aftermarket products can improve a pickup’s aerodynamics versus those that create drag. The automaker also said it madee a number of aerodynamic improvements to the design of its upcoming 2014-MY GMC Sierra full-size pickup truck.

GM’s Diane Bloch, GM aerodynamic performance engineer, said her team looked at how various aftermarket products affect vehicle dynamics in a pickup truck. They found that a tailgate in the “up” position is more aerodynamically efficient than in the “down” position. Next, her team found that an aftermarket net is worse than having a tailgate.

“Replacing the tailgate with an aftermarket net is worse than having no tailgate at all,” Bloch said. “Imagine dragging a solid object and a fishing net through water. The net is going to require more muscle.”

The team also found that certain accessories, such as bug deflectors, wider tires, or aftermarket bumpers can increase the coefficient of drag, worsening fuel efficiency.

A few accessories can improve aerodynamics, though, according to Bloch’s team’s findings. They include soft tonneau covers (versus hard truck lids), which help shape the airflow over the back of the truck, and running boards, either round, tube-style running boards, which can help smooth out airflow along the side of a pickup truck, or flush-mounted, which are better, according to Bloch’s team.

For the all-new 2014 Sierra, the automaker created a new air dam, below the Sierra’s front bumper, which reduces drag by directing air toward the ground and away from the truck’s underbody. Next, the automaker said the ducted flow path between the grille and radiator prevents air from “swirling” in the cavities in the pickup’s front. The Sierra’s tailgate and center-mounted stop light are also optimized to move air around the truck.