From "Click and Clack":
Dear Tom and Ray:
I read your recent article in which a reader (also named Kathy) asked whether a pickup truck gets better gas mileage with the tailgate up or down. You guys said "down." My husband and I made a very large bet about the very same issue. I work for GM as an engineering intern and had a chance a couple of weeks ago to tour the design facilities in Detroit. When I got to the wind-tunnel building, I asked the engineers this same question. They laughed and demonstrated that trucks are designed so the airflow creates the least amount of drag when it flows off the roof and past the tailgate in the upright position. They said that leaving the tailgate down would actually decrease a truck's fuel mileage. So guess who won the bet, guys? -- Kathy
Ray: We know who won the bet, Kathy, because we received letters about this from engineers scattered throughout the automotive industry.
Tom: Here's one that offers a more technical explanation for you (still) nonbelievers.
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Dear Tom and Ray:
I'm an aerodynamics engineer. When I was in the U.S. Air Force a few years back, I worked with folks from the Lockheed low-speed wind tunnel. In the 1970s, aircraft production went into a slump, and Lockheed started looking for other customers for its wind-tunnel services. Prime candidates were the auto makers, and Lockheed was successful in convincing Ford, among others, that the wind tunnel would help them reduce drag and wind noise on their vehicles. Needless to say, in the past 15-20 years, Lockheed has learned a lot about car and truck aerodynamics. Anyway, they actually performed drag tests on pickups with the tailgate both up and down, and found that drag was actually LOWER with the tailgate CLOSED! This ran counter to their intuition (and yours). The reason is that a closed tailgate sets up a large "bubble" of stagnant air that slowly circulates around the bed of the truck (we aero types call this a "separated bubble"). When air approaches the truck, it "sees" the bubble as part of the truck. So to the air, the truck looks like it has a nice, flat covering over the bed, and the air doesn't "slam" into the vertical tailgate. If the tailgate is open, or replaced by one of those "air gate" nets, however, that nice, separate bubble in the truck bed does not form (it "bursts"). Then the air approaching the truck "sees" a truck with a flat bed on the back of a tall cab. This is a very nonaerodynamic shape with a very LARGE drag. So, believe it or not, it's best for gas mileage to keep the tailgate CLOSED. Hope this information is helpful. Ed Fitzgerald, Research Assistant, Dept. of Aero/Mechanical Engineering, U. of Notre Dame
Tom: Sounds pretty convincing, Ed. Thanks. We also heard from none other than Bob Stempel, the former GM president, who wrote us to say that aerodynamically it doesn't make that much difference. But, he says, a pickup truck is structurally much SAFER with the tailgate up.
Ray: So for that reason alone, we suggest you throw away those tailgate nets, folks. And as your flight attendant might say, please return your tailgate to the upright and locked position.
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Trucks, Tailgates and Mileage: The Definitive Word?
From: Kathy Bretz
Tom and Ray:
I read your article on Friday, August 15, 1997, when another reader (also named Kathy) asked whether a pickup truck gets better gas mileage with the tailgate down.
Ironically, my husband and I have bet our bank account against each other on this very question. I work for GM as an engineering intern and had a chance a couple weeks ago to tour the design facilities in Detroit. When I got to the wind-tunnel building, I asked the engineers this very same question.
They laughed and demonstrated that trucks are designed so the airflow creates the least amount of drag when it flows off the roof and past the tailgate in the upright position.
With the absence of a tailgate, or when the gate is down, the drag begins at the cab, and not at the tailgate. So, leaving the tailgate down would actually decrease a truck's fuel mileage due to larger drag.
Now, Tom, not to give you too hard a time about your "vertical wall" logic, but you better let your brother Ray guess which one of us won the bet!
