Does it save gas to drive with your tailgate down?
This was one of the featured tests done on the Mythbusters TV progam a couple of years ago, and IIRC, they found that this trick did not increase your gas mileage.
ADDENDUM:
From the Car Talk archives, circa 1997–
[b]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. [/b]
As with all things aerodynamic it depends. Some trucks will increase consumption and some decrease, others no change. The only way to know about your truck is to try it out and measure carefully. Of course once you put stuff in the bed, it may change.
My guess is that on the average it is going to have less fuel consumption with the tailgate UP.
WOW ! Nice info, can’t wait to smugly argue that one in my local bar 
Of course no-one will believe me, they all drive around with open tailgates around here.
Well, they probably also still believe that we were attacked by Iraq on 9/11, so I would suggest that you not put much faith in any of their beliefs. Bad information tends to stick in people’s minds, even when confronted with reality.
What about the weight saving by taking the tailgate off? Probably as small as the gate up/down issue.I propose a standard: How much MPG must be achieved before a action can be said to be one that gives better MPG A action that gives you .25MPG better is just bar talk. (I am not saying set the standard a .25 but lets make a number).
There was also an episode of mythbusters where they did this. With two identical pickups driving the same route and with all acceleration being done with the cruise control, the truck with the tailgate UP went an extra 25 miles or so.