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Tailgate Up

I think the studies show that the air gets trapped in there and makes it's own barrier that allows the stream to come over the bed and slip behind the gate. When the tailgate is down it doesn't allow this to happen and it causes more drag on the vehicle.
This is correct. I watched a wind tunnel test on this. The initial air creates a mini vortex just behind the cab. The wind just flows over the vertex as if there were body panels there and continues on. The gate down causes the wind to curl up back onto itself as it tries to create a big vortex. The wind coming up hits the wind trying to pass over the truck and actually causes resistance. It was pretty cool to see how it works.
Of course, this only matters if you are going fast enough. If you're drag racing, the run is so short that it may not make that much of a difference.

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