I have a 2004 Dodge Ram 1500 quad HEMI.
1-leaving the tail gate increase fuel efficiency?
2- Does covering the load area with a shell or canvas/plastic cover reduce air friction and increase mileage
From what I have heard…
1 - No.
2 - Yes.
I believe if you cover the bed it will increase fuel efficiency.
Keeping the gate closed creates a vortex in the bed which allows the air to flow smoothly over it and off the back. (One of Mythbusters’ more scientifically valid studies.)
The only way to increase the efficiency of this truck is not to drive it…
With this truck, I’d consider tail gate, shell and canvas cover based upon needs that have nothing to do with fuel efficiency.
I can’t say whether the bed cover can help, but it doesn’t seem to hurt enough to worry about. It does seem to cut down on wind noise. Tailgate down will not help enough on your truck because your engine is a gas guzzler anyway. My GMC is rated for the same 14 MPG overall but I get about 18.5 on the highway with 4WD auto, AC, PS, and extended cab with P245/16 tires and 4.8 engine. I consider that to be good mileage. A camper shell will hurt by at least 2MPG on your truck, possibly more.
The tests that I’ve seen on these issue and the test that I myself did have show no significant effect. Tests in a widn tunnel have shown Zeppflyer’s vortex as the reason.
My own test, while I’ll freely admit was not a controlled or comprehensive lab test, was thus. I used to commute 103 miles each way, almost all highway mileage, in a (at the time new) 1989 Toyota pickup. During the summer, I tried running for one month with the tailgate up and one month with the tailgate down. I found zero difference in mileage.
The cab sort of acts like a windshield for the tailgate, thus there is not a strong wind there for the tailgate to hit. If you have ever ridden in the bed of a pickup truck at highway speeds, you may have noticed that your hair gets blown forward, just like it does behind the windshield of a convertable. Loose trash in the bed of a pickup truck also tends to get blown toward the cab instead of toward the tailgate.
So, it seems logical to me that it doesn’t make any difference tailgate up or down.
Maybe… IF that cover has a low coefficient of friction. There is a reason hard cover roof top carriers get better fuel economy than soft roof top covers. If the cover is made of canvas, like the OP mentions, or even a plastic tarp, I would expect a drop in fuel economy.
The trash heads straight through the slider…
I’ve seen footage of wind tunnel testing. The air in the bed actually forms a laid-down vortex with the top moving toward the rear and the bottom moving toward the slider. With the tailgate up, the vortex fills the bed, the front extending to the top of the cab. With the tailgate down, the vortex simply forms more of a triangular shape, with the top laminar flow moving from the top of the cab to the edge of the lowered tailgate. In neither case is there resistance for the air moving over the top of the cab and bed. The air moving over the roof simply slides over the surface of the vortex and gets left behind. Opening the slider allows some of the air coming forward in the bottom of the bed of the truck to spill into the cab. It’s producing higher pressure than the air in the cab.
The air pressure testing also showed a small low pressure area behind a raised tailgate, but it was of no significance.
I should disclose that I saw this wind tunnel testing many years ago. Today’s huge (needlessly oversized, enormous, ridiculous, exaggerated, testosterone-laden, ego-compensating) pickups may have different aerodynamics. Um…is my bias showing?
Funny. Does the same reasoning apply to a Hummer? I want to get my H1 up to 30 MPG 
If you idle it around town and never hit the gas pedal, you could. 
And if you do have to drive it make sure you are always heading downhill, with a tailwind, towards a magnet factory.
I have observed this vortex myself. I once accidentally drove off with an open box of styrofoam peanuts in the back of my truck and once I hit the highway they all got sucked out of the box, but they all just circulated inside the bed without leaving it.
GJ, was that salted or unsalted ?
Greasy, you did not say that the tailgage was open, do you suppose the peanuts would have left the bed if the tailgate was open??? I tried to read mountainbike’s explanation but couldn’t understand it.
The peanuts would have circulated forward along the bottom of the bed, up the back of the front of the bed, and rearward from the top rear edge of the cab to the edge of the tailgate. Since I’ve experienced this myself with lightweight dry materials like bark chips, I can testify that there would be some spillage over the edge of the tailgate, but most of the peanuts would have kept circulating within the cab.
If the slider is open some of them will come right into the cab from behind. I’ve had that happen too.
Yeah it was with the tailgate closed and as far as I know unsalted (although maybe those rice-based ones you can eat might taste okay with a little salt). I don’t have a slider so I couldn’t check that variable.