Many pickup truck owners believe that driving with the tailgate down improves aerodynamics and thus boosts gas mileage. After all, they reason, the wind comes over the cab and hits the tailgate as if it were a wall, slowing the vehicle and creating drag.
Although it seems counterintuitive, it is not more fuel efficient to drive with the tailgate down. Engineers and designers have discovered that leaving the gate closed creates a large bubble of air in the bed called a "separated bubble." This bubble of trapped air acts as a cover over the bed and improves vehicle airflow. Leaving the gate down increases turbulence and drag by eliminating this bubble of air and literally pulls air in from the open gate, acting as a brake.
To improve a pickup's fuel economy, the gate should be left up and a hard cover should be added over the bed. A cover will duplicate the effects of the bubble and even slightly improve them. Regardless of truck size, fuel economy will show immediate improvement over non-covered, gate-down driving. Avoid open, web-like fabric tailgates that only increase drag.
There are other reasons to not drive with the gate down. The tailgate hinges are not designed to sustain the repeated shocks of prolonged open driving; they can be damaged irreparably from doing this. Having the gate down also reduces the design safety benefits of the rear bumper, as the gate will be struck first in most accidents. Replacement gates are expensive, and rear impacts could also damage the truck's framing by forcing the gate up and into the bed.
This very topic was the subject of a November 2005 episode of the Discovery Channel's "MythBusters" program. In that episode, the show's team conducted numerous tests on two identical pickups driven side by side at the same speed until one ran out of gas. Of the two, it was the tailgate-down pickup that lost the contests.
Pickup truck owners are slowly catching on to the need to keep their tailgates up to improve fuel economy. However, until this misconception is completely dispelled, the appearance of tailgate-down vehicles on the road will continue.
