> I drive with the tailgate down when I use the pickup unloaded.  It
> does save fuel because up, it can scoop up enough air pressure at
> 50 mph to cause significant drag

Mythbusters claimed that with the tailgate down, the vortex behind
the cab disappears. According to the pick-up manufacturer (smoke
trail test film), air coming over the cab roof bounces off that vortex,
resulting in less drag

http://www.cartalk.com/content/columns/Archive/1997/October/05.html

As they say there, it's counter-intuitive

Not that I'm doubting your results, you know your vehicle and
have the numbers

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