5er tailgate or not for better mpg?

I'm no aerodynamicist, but I'm sure there's lots of turbulent air that comes off of the 5th wheel and enters the bed. Turbulence = drag. If I had to guess, I'd say that the louvered or missing tailgate would only enhance the turbulent air flow. If there's actually a measurable difference I'd imagine that it would be very small. The front of the truck, gap between the trailer, and the trailer's rear end would have the most significant impact. If you could fashion some kind of boot to prevent the air from getting into the bed in the first place, then you might have some useful increase.

If you're a fan of Mythbusters: Tailgate up or down

If you drive with the tailgate down keep in mind two things:

1. The tailgate sticks out past the bumper and bends easily if you back into something.
2. The right bounce when you're driving could leave your tailgate dragging behind you by its retaining cables.

Many scientific experiments have been made regarding truck beds and tailgates. Here is the latest. OEM tailgate up is typically best. Any difference in fuel consumed is insignificant. So drive with the tailgate down when you have large loads and please tie cargo down. Use a bed cover of whatever design you want based on how large and secure you want the cargo covered. Don't worry about fuel.

I think the problem is there are too many variables:
- Long or short bed
- Standard or extended pin box
- Lower riding 4x2 vs higher riding 4x4
- is the trailer riding level or nose high
- Is the bed otherwise empty or full of stuff
- Wind conditions (for, against, crosswise)
- Style of 5er front end (low, mid height, tall along with squared off or rounded)
- Dually or single rear wheels
- Regular cab vs crew cab

All of these things affect wind resistance in some but not all situations. If you are worried about fuel economy, go a little slower and it will almost always result in better fuel economy (even then you will get people arguing they get better milage at 75 over 55 because the gearing puts the motor in it's ideal rpm range, though I'm not buying it when wind resitance increases with the square of the speed)

Years ago with another truck I owned (GMC 2500 gasser), I tested driving with the tailgate up and down to see if there was a difference. I was doing a 500 mile round trip down here in FL, so no mountains to deal with. I used cruise control as much as possible and drove the speed limits. I drove down with the tailgate down and back with the tailgate up. I actually got worse mileage with the tailgate down by about .5 MPG hand calculated. I later added a soft tonneau cover and had to make the same trip. Got about .5 MPG better than with the tailgate up.

Certainly not scientific, but at least indicates very little difference and possibly will be worse. It would probably take a while to recover the cost of adding a different tailgate.

I have seen numerous claims of ways to improve pickup mileage with bed/tailgate changes over the years. These would include with or without tonneau covers, with or without tailgates, with tailgate up or down, and with the louvered 5'er type tailgate. Without exception every report I have ever seen indicated absolutely zero difference in economy. Sooooo, I have a soft roll up tonneau and standard tailgate. Works for me.