I did some quick looking, and although I didn't find the article I remember, I found so much other information pointing to the aerodynamic DISadvantage of running with the tailgate down.

This is an article from Road & Track: http://www.roadandtrack.com/article....;page_number=1

Here's one from TruckTrend.com: http://www.trucktrend.com/features/c...yth/index.html

And some discussion on Snopes.com: http://msgboard.snopes.com/cgi-bin/u...3;t=000459;p=1

One disadvantage (besides increased drag) of running with the tailgate down or removed is the lack of bed rigidity. Without the tailgate in place, the sides of the bed are standing up "on their own". That tailgate completes a link between the bed sides, and adds rigidity to the back of the truck (especially in an accident).

Most sources said that a tonneau cover or clam shell is the best thing you can do to improve economy. It will genuinely reduce drag without adding a lot of weight.

 

Mythbusters did a thing on this. The wind like swirls behind the cab with the gate up.

 

It looks like the tailgate would be a huge hinderance, but with it up, it creates a pretty stagnant bubble of air "in" the bed. This is demonstrated by leaves that blow and tumble in circles in the bed but don't generally fly out. Wind doesn't fall down behind the cab and then back up over the tailgate...it flows pretty linearly from the top of the cab to the top of the tailgate. Lowering the tailgate, or removing it altogether, creates turbulence by disturbing this airflow pattern.

 

I also have noticed that with the gate up, the air in the bed seems to swirl over the cab up to the front of the bed almost making an invisible bed cover. I used to havea soft tonneau cover on my truck but it ended up falling apart so i just took it off. I would like to get a nice fiberglass cover but they are sooo expensive.

 

from what i have seen, most of the testing is done on very new trucks that are more aero than older trucks, people with older trucks, like from about 2000 and olderthese tests might not apply...who knows?

i have noticed that when my bed is wet that onlythe back half dries from the wind. i assume that air is coming over the top and down into the bed to dry the back part of the bedand catching on the tailgate. i havent tried running a tank of gas with the gate down so i dont know if it helps mileage or not. i wont be trying this anyway since i worry about idiots hitting my tailgate. i am getting a tonneau anyway, not so much for the mileage, but for the looks and security.

 

Yup the article I read (can't remember where I read it and I know MythBusters did a show on it) said that you actually can get better mpg with no tonneau, but you get the worst mpg with the tailgate down and it can mess up your tailgate.

 

mythbusters used a newer ford truck...not sure on the year, something like an 06, which is designed more aero than older trucks. im not sure, but im betting that the manufacturers started designing trucks more aero just a few years ago and these tests dont apply to older trucks. tonneaus do help, testing shows this. maybe not on new trucks, but ill bet it does on older trucks that were not designed with mileage in mind.

 

The article I read used a 1994 Dodge Ram. If you look at our Dakotas, a lot went into the aerodynamics. Look at the sculpting of the mirrors, and even the small "bump" spoiler in the tailgate. Same as the small bump in the back of the top of the cab. Aerodynamics are everywhere, to maximize economy. This is not new.

 

Don't get me wrong, I am putting a cover on mine and a locking tailgate handle, but in one of the testing articles they did say people lost like 1 mpg with the cover on and lost like 3-4 with the tailgate down. I'll try to find the article and post it.

 

this makes sense to me...the turbulant "air bubble" in the bed will force air to fly over it and then over the tailgate...but doesnt it make sense that a the so called "air bubble" would cause drag in the first place? a tonneau it seems would eliminate all drag and the air will come over the cab and over the smooth tonneau causing no drag at all. just a thought.