UP or DOWN?
Which position actually produces the least aerodynamic drag?
In the last 15-20 years, Lockhead Aircraft has been using it's low-speed wind tunnel to test the aerodynamics of automobiles. The following was taken from an article in the San Diego Union/Tribune Oct 25 1997:
"...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...
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 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 non-aerodynamic 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."
This information was written by: E.F., Department of Aero/Mechanical Engineering, U. of Notre Dame
Here are two other university studies that have illustrate the same results:
http://amos.indiana.edu/library/scripts/tailgate.html