One problem with the argument concerning the tailgate is that different trucks have different aerodynamics. One may do tests on one's own truck and then come to a general conclusion, which may or may not prove to be erroneous.
I have personally tested the "lower the tailgate" theory on two pickup trucks; both picked up mileage in the realm of 5 to 8 percent. Not a lot, but certainly measurable. When a cab-high shell was placed on top of the bed, another 12-15 percent was realized.
However, on one specific truck I have kept good records and done before and after testing on a number of modifications. The best modification on this truck was to REMOVE the bug shield.
I started with a 1968 Ford 360 A/T with a bug shield. As things wore, replaced the bias ply tires with radials, replaced the dogmatic trans with a 4-speed, replaced the 360 2 barrel with a 390 4 barrel, removed the bug shield, added dual exhausts (tried headers for awhile, huge mistake), and now drive with the tailgate down (I had a shell on the truck but have now removed it). Mileage at 70 MPH has increased from 9 MPG to a little over 17 MPG with these modifications. Others may find different results. The two best individual gains were by removing the bug shield (about 2 MPG) and junking the dogmatic in favor of a 4 speed (about 2 MPG). Truck has highway gears (I don't know the exact ratio).
I have built a set of dual 4 barrels to put on the engine if I ever get reasonably caught up in the shop.
As mentioned earlier, these changes may or may not help others, depending on the aerodynamics of the truck.
Jon.
