Drag ?

IBcarguy wrote:
How would you ever know? Wind tunnel test? I'm no authority on this topic but I think it would be negligible and maybe not even measurable. I guess you could try checking mpg with it open compared to closed. There are a lot of other things that would impact MPG rather than your sliding bed cover.
The wing, however might help especially if your 5th wheel cap is rather flat. Mine is very rounded and curved and I doubt if a wing would reduce wind resistance very much.
I think slowing down to 55mph would have the greatest impact on fuel consumption.

I appreciate the reply, and fully understand your point, however a study at Institute of Fluid Science Tohoku University 2-1-1, Katahira, Aoba-Ku, Sendai JAPAN, Using a wind tunnel showed that a short bed truck had little effect on drag and a large bed truck created a lot of drag while and driving with the tailgate down was even worse for both vehicles. The Geometry of the Truck Bed also made a difference, but as most truck beds in the USA are within the same geometry field I don't see a big comparative difference.
I'm my scenario, I am in essence reducing a long bed to a short bed, but the area behind the cab to where it becomes a short bed is a question I don't have an answer for, nor does this study give any idea of how the drag is increased or decreased when a trailer is attached.
The wing, as has been stated many times does work, If it didn't the Commercial Industry would not be using it, Plus assuming all other aspects are in tune and maintained the distance between the wing and the trailer is the biggest determining factor of efficiency. I agree that a cap with a curve is more stream lined than a brick wall cap, and there are caps with contorted shapes creating a stream line effect. But again the distance between the back of the truck and the cap on the trailer is an unknown..
In theory it should reduce drag I guess trial and error will provide the answer...

Thanks