Keep it up

We've all seen owners of pickup trucks driving with their tailgates down. But does lowering the flap at the back of the truck improve aerodynamics?

We've all seen owners of pickup trucks driving with their tailgates down. Lowering the flap at the back of the truck must improve aerodynamics and save fuel... right?  Wrong!

In fact, driving with your pickup's tailgate down or off actually does the opposite most of the time, said Kevin Cooper, a researcher at the National Research Council (NRC) of Canada, who has spent more than 35 years testing the aerodynamics of countless vehicles and other structures in NRC wind tunnels.

Cooper demonstrated that the tailgate-down myth was just that - a myth. Back in 1999, in between other tests in NRC's 9m wind tunnel in Ottawa, Cooper studied the aerodynamics of different makes and models of pickup trucks.

His team's tests included trucks with different box lengths. They found that the shorter the box, the worse the drag becomes when the tailgate is left down or removed.

While lowering or removing the tailgate on trucks with an eight-foot long box may decrease the drag by a small margin (less than one percent), lowering it on a six-foot cab - like those typical of crew-cab trucks - increased the drag by five percent. Using a mesh tailgate cover raised it another five percent or more.

'The end result is that having a closed tailgate is not nearly as bad as people think,' said Cooper.

Here's why: Most of the drag on a road vehicle comes from its backwards-facing surfaces - on a pickup truck, these are the back of the cab and the tailgate. Some people try taking the tailgate off completely to eliminate the small amount of drag on the tailgate. Unfortunately, this increases the drag on the back of the cab, and they end up worse off than if the tailgate was left on.

Auto companies spend a great deal of time and money wind-testing vehicles, making sure air flows just above the tailgate of their pickup trucks. Lowering the tailgate throws all that effort out the window, Cooper said.

Cooper encourages truck manufacturers to let their customers know that the trucks are most aerodynamic with the tailgate up. The best way to further suppress your pickup's appetite for fuel is to add a fibreglass or vinyl tonneau cover. These covers reduce the air flow that circulates behind the cab - lowering drag for an even more fuel-efficient ride.