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Tailgate up or down
The answer is simple. Keep your tailgate up. I did a series of wind tunnel tests on full-sized pickups in the 30-ft-wide, 30-ft-high test section of the NRC Low Speed Wind tunnel in Ottawa, Canada to investigate this issue. The results almost always showed that the tailgate up was best. With an 8-ft box, having the tailgate down or off decreased the drag coefficient (CD) by about 0.003 in a CD 0f 0.50. With the shorter boxes found with crew and extended cabs (6 ft & 4.5 ft), the drag coefficient was increased by up to 0.025 when the tailgate was of and a little less with it down. Adding a mesh tailgate instead of a solid one increased the CD by another 0.025, to a total of 0.05 above tailgate up. I would expect that any similarly porous tailgate would also increase drag. Since the gain of 0.003 is extremely small, it would not seem worth doing considering the loss of utility. Putting the tailgate down is a real loser since it is unsafe to following traffic and increases drag.
There you have it: keep your tailgate up.
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