By G.R. Whale
At the SEMA show in Las Vegas, the ultimate big-box store for gearheads, vendors display a $35-plus-billion pie of filters, lights, exhaust pipes, and blinding amounts of flash. Hidden among all this glitz, perhaps due to their utilitarian nature, are truck tonneaus and caps, which constitute a significant portion of floorspace and aftermarket dollars. A perspective buyer need only hear two words to consider getting one: fuel economy.
Even before Audi's late- 1970s groundbreaking 100 and the later ellipsoid Taurus, auto manufacturers knew that lower aerodynamic drag was among the most cost-effective ways to increase fuel economy. It especially helps a vehicle's highway number, since aero drag rises exponentially with speed. The easiest way to make a pickup more aerodynamic (presuming there aren't items like a 12-inch lift or an overhead light bar and rack in place) is to cover the bed, just as is done to every Avalanche, Escalade EXT, and Sport Trac before they leave the factory.
In a previous issue of Truck Trend, we ran a column busting the myth about dropping a tailgate to lower drag. Since then, we've gotten letters that claim at least one pickup truck went slightly faster with the gate down than when it was up. Readers went as fast as possible out in the open with the gate up, then repeated the run with it down. No mention was made of any instrumentation for verification, changes in weather, or even the engine loosening up (don't laugh, it could happen on older-model trucks with 50 miles on them).
My own theory is simple. With a pickup tailgate dropped, drag and rear lift tend to increase, in which case there's more rear-tire slip. This causes the speedometer to read faster than the truck is actually going.
Theory isn't enough to win a beer bet, however, so I chased down a few engineers (who shall remain nameless to protect their pensions) to get their insights. One claims that in dirt-truck racing, before the Craftsman/NASCAR series existed, dropping the gate made no difference to lap times, but the bed welds always broke sooner. Another says the majority of drag occurs in the back half of the bed, and even a half tonneau at the rear helps but looks odd, and dropping the gate doesn't help. But both serve as reminders that the tailgate often aids box rigidity, which is why prerunner builders always brace the bed sides.