Fuel Economy 101
CLOSING THE TAILGATE DOWN MYTH
Tailgate up, or down? A few facts to boost your pickup’s fuel economy numbers
By Bruce W. Smith / ©2006 Truck Test Digest
Tailgate down. Tailgate up. Which gives your pickup the best fuel mileage?
If you look around at the number of pickups driving down the highway, one would believe the answer is as obvious as the tailgate itself: tailgate down.
After all, the wind rushes over the cab and hits against the tailgate just like it were a metal wall, slowing the truck down in the process.
But what may look obvious to the everyday pickup owner is not what the automotive designers and aerodynamicists see when they place pickups in wind tunnels to examine the effects on fuel economy.
According to such experts, the tailgate left in the up, or closed position provides better fuel economy than when it’s open.
The tailgate in the closed, or up, position actually creates a large bubble of air in the bed that aero engineers call a “separated bubble” or “Locked Vortex Flow.” This is a very good thing.
This invisible bubble of air trapped inside the open bed box actually helps make the pickup more aerodynamic. In essence, this bubble of air trapped in the bed by the tailgate acts like an invisible tonneau cover over the bed box.
“Putting the tailgate in the down position tends to increase turbulence and drag of the open box” said Jack Williams, one of Ford Aero Systems Engineering team when asked this question a couple years ago during an F-150 test drive. .
“We’ve seen drag increases of between .5- to 1-percent when a pickup tailgate is left in the down position,” said Williams who spends considerable time testing vehicles in the most sophisticated wind tunnels in the world.
“Those flow-through, web-like fabric tailgates tend to increase drag even more; we’ve seen increases as much as 4- to 5-percent.”
As any savvy vehicle owner knows, more drag translates into less fuel economy. And, pickups have a lot of inherent wind resistance just by their shape.
“Air flow doesn’t like making 90-degree turns, which is what naturally occurs at the back of a pickup cab,” says Dr. Edward Fitzgerald, an aerodynamics engineer with a major airplane manufacturer.
Fitzgerald says such airflow creates a lot of turbulence, turbulence that creates a suction effect that literally pulls air in from the open tailgate, and in effect, acting like a brake.
Dodge aerodynamic engineers, like those at the other pickup manufacturers, agree.
“Our wind tunnel data falls in line with industry conclusions – that the tailgate down configuration is significantly higher in drag than the tailgate up (around 2-3%),” says Mark Gleason, Supervisor / Aerodynamics at the DaimlerChrysler Scientific Labs and Proving Grounds.
“Although it seems counter intuitive, having the tailgate up acts somewhat like a deck spoiler on the trunklid of a race car. The tailgate actually raises the pressures on the rearward facing surfaces of the pickup cab and the forward surface of the bed, thereby lowering the drag of the vehicle,” explains Gleason.
“Testing conducted by SAE confirms this condition, estimating an increase in vehicle drag of 2-3% with the tailgate down. Testing also shows that, counter to advertised claims, mesh tailgate replacements will degrade vehicle drag even further.”
Always looking for even minor ways to reduce vehicle drag and improve fuel economy, Gleason says Dodge has worked to optimize the benefts of the tailgate being up (in the closed position) even further by placing a tuned spoiler on the top of the Ram tailgates for the 2006 model year.
“Dodge aerodynamicists recommend that the drivers of our pickups take advantage of the aerodynamic improvements derived with our tailgate mounted spoiler by leaving the tailgate up,” says Gleason, “or by using a tonneau cover if suited to their use of the vehicle.”
Aerodynamicists who spend a great deal of time studying airflow, say the small increase in drag also increases with speed.
For example, a .3-percent increase in a full-size pickup’s drag with the tailgate down has much more adverse effects at 70-75 mph than it does at city speeds.
The size of the pickup really doesn’t matter; compacts are affected just as much as their full-size brethren.
Toyota engineers say that although they haven’t done any EPA testing, they speculate dropping the tailgate can actually decrease fuel economy depending on the cab and bed configuration of the pickup.
“Air flow coming over the roof rolls into the bed creating more drag than when the tailgate is in the up position,” says one body design engineer.
A segment on the Discovery Channel’s Myth Busters (Episode 43, Nov 16, 2005; www.discovery.com) showed the real-world results of the tailgate up/down myth. They conducted numerous tests, including one in which ran two identical pickups driven side-by-side at the same speed until one ran out of gas.
The loser? The one with the tailgate down.
According to Warren Spears, a 40-year veteran body shop owner and pickup/SUV repair expert in Long beach, Mississippi, another concern with having the tailgate down is the damage that is done to the tailgate hinges.
“The hinges are enduring the same pounding while you’re driving as they would slamming the tailgate closed repeatedly for whatever amount of time you’re driving down the road,” he says. “The bumpier the road the worse the hinges get abused.”
Spears says replacing tailgate hinges in today’s pickups runs about $30 a side. An even bigger concern he says is damaging the open tailgate.
“Driving around with an open tailgate is asking for trouble,” says Spears. “Instead of someone tapping your truck’s rear bumper, they ding the tailgate—and tailgates ding easily.
“A bare tailgate costs $400-$500. Then you have to add in the cost of painting and clearing, which can easily add another $150 to the repair cost—and that’s if you were lucky enough the tailgate didn’t jam into the bed and bed sides.”
The best answer to improving a pickup’s fuel economy is to leave the tailgate up and cover the bed with a tonneau cover—a hard one works better than a soft version. But both contribute significantly to overall fuel economy.
“Tonneau covers on pickup boxes do reduce aerodynamic drag,” says Williams. “We’ve seen reductions of about 8- to 10-percent on the F150. The average fuel economy improvements for EPA city/highway cycle should be about 2-percent. The average steady-state (cruise control) fuel economy improvement at highway speeds should be about 5-percent.”
Such a reduction in the pickup’s drag could be translated to the equivalent of a free gallon of gas for every 20-gallons burned. Put another way, the owner of a full-size pickup could save about $5 per fill-up.
I’ve tested pickups and SUVs professionally for more than 20 years and have run my own real-world numbers on the tailgate issue.
Those tests included driving full-size pickups 100 miles on flat interstate on days with no prevailing or tail winds. The speed was kept at a constant 70mph with cruise-control engaged. I tried it with tailgate up and then a run with it down, refilling at the same gas pump each time.
My observations each time have been a loss of about a half-mile-per-gallon in fuel economy when driving with the tailgate down. Put a tonneau cover on the same trucks and the results were a gain of about 1mpg.
Even the boys who design the NASCAR Series pickup apparently feel there’s merit to tailgates being up and beds covered.
Tailgate up, or tailgate down? If you want save money as a pickup owner and impress your pickup-owning buddies with the fuel economy numbers, keep that tailgate closed and cover the bed. – Bruce W. Smith