I think the tailgate and tornado have been so beat to death, it isn't even worth discussing. It has been well established for years that lowering the tailgate does NOT improve milage and that the tornado is a joke that does nothing.
That is right about the air pocket and the test was done on an F150. The Tornado does give me like an extra mile or two to the gallon. When I bought my truck, I could go like 280 miles before the tank was empty. Then I got a tornado and and K&N cone filter and took out my air box and I was getting to like 330 miles. I think those numbers work for me :)
After I put my cap on i'm averaging 1.5 miles to gallon.. 28.5 was 27 before. You can feel a difference when going down the highway, the wind doesn't seem to pull it down as much. Tailgate up or down rely's on long or short bed and the aerodynamics of the truck. On the 2000 f150 I had the mpg dropped with the tailgate down... on the Ranger I never tested it so don't know. But the aerodynamics of the ranger isn't near and good as the F150.