Originally Posted by Rugged Brown
I use a tonneau cover when not towing and it is quite apparent that a wind tunnel plummets the back 1/3 of the cover. This leads me to think that an open tail gate will allow for less resistance and improved gas mileage. With a fifth wheel in tow, the aerodynamics may be completed different. I posted that pic because I saw a guy with it and thought it was rather ingenious using plumbing pipe to fabricate the tail gate. The pic was intended for humour (Canadian sp.) only.
I also use a tonneau cover when not towing and from the mid rib back there is quite a bit of wind that hits there. To the point that I slide a large piece of cardboard under the tarp and over the ribs.
I seems counter intuitive for that much buffeting of the cover to not actually enter the truck bed. In fact, it would if you had an air gate and you had nothing in the bed the airflow would be smooth and exit at the truck bed level. However, nothing is so neat.
With the tail gate installed the air that would exit is trapped by the tailgate and the airflow will not actually enter the bed more than an inch or so. The reason the tarp of the cover gets beaten up is that the air inside the bed is not allowed to pressurize (like filling the bed with water) and the cover acts like "wind ripples on the surface of the water".
If you have stuff (fuel cans, hitch, and egg crates full of gear) in the bed, the airgate actually encourages aerodynamic drag by allowing that fast moving air to enter the "dirty" floor of the truck bed actually increasing drag; thereby negating any benefit gained by allowing the air entry into the bed in the first place.
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