We heard from a lot of experts about our assertion that a truck's gas mileage would improve with the tailgate down.

Dear Car Talk

Dear Car Talk | Oct 01, 1997

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)

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

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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