Just read a very informative article today in Road&Track magazine.
The article was about aerodynamics of trucks, and it answered the age-old question: tailgate up or down?
I see this debate pop up on here about once a month, so I paid special attention and "took notes"....
It presented several values of Coefficient of Drag (CD) based on the different scenarios. The results were as follows:
Test Vehicle: 2002 F-150
CD with tailgate up: ~51.2
CD with tailgate down: ~52.5
CD with net style tailgate: ~55.5
CD with partial tonneau: ~50.7
CD with full-sized hard tonneau: ~49.7
Basically, the higher the number, the more drag the vehicle produces through the airstream. A relatively small CD will also yield better gas mileage given an identical test vehicle and similar testing situations. So a tonneau DOES help with mileage and aerodynamic efficiency after all. And leaving the gate up is the best "free" thing you can do to improve CD of your truck.
Aside from the main slipstream being affected, the gate being down was also proven to decrease efficeny of air flowing under the truck, as well as making the box less rigid and more prone to major damage during a wreck, should it occur.
If anyone want's to see for themselves, look for this month's issue of Road&Track magazine.....it is in the back 1/4 of the book.