There is a low pressure area behind the cab, caused by the airflow flowing around and over the body work. As the airflow meets behind the cab, it starts a buffeting just in front of the tailgate. The tailgate blocks its rearward path, induces some drag and causes a slight loss of fuel mileage. The tailgate also induces its own low pressure area behind the rear bumper, and causes additional drag.

A louvered tailgate is supposed to allow the airflow to continue with less abatement, and reduce the amount of disturbance caused by the blockage. Web gate replacements, or removing the tailgate also mitigates the airflow problems.

No real benefits at all in my opinion.  Before I bought our last truck, we always had trucks with tailgates and I was always doing different things trying to get the best fuel mileage while unloaded and the difference between having the tailgate vs. no gate or vented gate is with in 10ths... 

Something to think about is even if you have a vented tailgate or no tailgate you will still have bedmat problems when towing a gooseneck!  The trailer is going to act just like the tailgate at hwy speeds!.....

If the bed mat is your worry, I would rather spend the hard earned money on a spray in bedliner and use the mat to stand your horses on when you wash them.

Real world... Small effects on aerodynamics.  But if given a choice, keep the tailgate up.  The tailgate provides substantial structural support to the truck body. 

I wouldn't spent the money on a vented tailgate.

Here's aerodynamics from the SAE  http://papers.sae.org/2004-01-1146/

"...The aerodynamic effects of the pickup truck tailgate are examined in this paper. It is shown that the removal or the lowering of the tailgate increases the aerodynamic drag of a pickup truck, increases lift by up to sixty percent and increases the yawing moment. All these changes are negative and reduce vehicle performance, albeit, only by small amounts. This finding demonstrates that the commonly seen removal of tailgates to reduce aerodynamic drag is a public misconception that should be discouraged by manufacturers and by regulators..."

I think NOT.  A closed tailgate adds structural support to the bed.  Try twisting an open box versus a closed box.  The bed is attached to the frame.  So a closed box (tailgate closed) adds to the frame rigidity and strength.  Besides,  stuff one leaves in the truck bed falls out if you remove the tailgate.  My buddy found this out the expensive way

The best one is after you uncouple the goose hitch, get in and drive off, you then feel a thump, hear a crash and realise your tailgate is still up. The worse part is trying to explain to your wife, how it was she, who had to have left the gate up. The LQ then becomes your next living space. It beats the dog house.

So a closed box (tailgate closed) adds to the frame rigidity and strength

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Here's what the experts say:

http://www.propickupmag.com/tailgate-down-myth/

I agree with the experts, HOWEVER they do NOT address towing a GN or 5th wheel trailer and the impact of areodynamics with a closed gate vs a open one. 

Ha Ha Ha!....Sorry, I just had a visual! Truely, it isn't funny.....

I've been pulling gooseneck trailers since 1997 and almost NEVER leave my tailgate on. If I put anything into the bed while I am towing, which is rare, I put a net across the back. I have never had a problem with the truck flexing or not being stable due to the lack of tailgate.

My question would be why would manufacturers make tailgates removeable if taking them off gravely effected the structural integrity of a truck??? Sounds like they would be really opening themselves up to multiple lawsuits due to the accidents caused by lack of tailgates on trucks. I know I see this listed as the cause of accidents all the time- NOT!!!

I learned a lesson one day years ago. My tail gate was off and Loaded the truck and GN.  Put 3-4 bales of hay in the bed of the truck and headed out.

Going down the freeway, I feel a bump and look in the mirror and see a green cloud behind the trailer. A 70lb bale of hay had blown out and gone under the trailer. When it hit the axles it had come apart.   It created a serious cloud that made it almost impossible for other drivers to see through.

Lesson learned. Don't drive without some kind of gate. Even fairly heavy items like full bales of hay will blow out.

Mythbusters proved the fuel mileage thing about keeping your tailgate up too.

I have never seen a vented gate that was low enough in the center to hook up a gooseneck without having to let it down.  5th wheel hitch will clear, but a gooseneck?

They did not, but I did in my post!  Your "areodynamics" will be NO different while towing a 5th wheel or gooseneck with or without the tailgate!  The wind or air coming around and over the cab of the truck is going to hit the lower front of the trailer any way!  Test it for yourself, get some sort of paper, floor mat or something and hang it under the gooseneck above the jack so you can see it.  Then drive around with the tailgate on and off and watch the movement of whatever you hung up and you will see there is not going to be any "areodynamics" difference between them....

Most goosenecks and 5th wheels are like pulling a barn door around with you any way.......

So from reading this thread, I assume that quite a few folks assume that hauling something in the bed of a truck un tied down is ok without a tailgate??    I also assume that a few people have had things BLOW OUT OF THE BED when the GN is hitched up although they did not have that problem when the trailer was not being towed.   

   A simple deduction on my part would be that turbulence caused by the trailer is blowing heavy objects out of the bed of the truck..  Is this not a high pressure area caused by air flow off of the front of the trailer?    Does the air coming over the cab of the truck deflect down and intensify due to the spillage of air from the relitivly flat nose of the trialer??   

   I have had my aluminum trailer leveling ramps (about 20# each) nearly blow over the tailgate when the trailer is being towed.   Is this not a indication of high pressure and high wind speed?    When the trailer is not behind the truck the ramps will ride in the same spot at well over 90 miles per hour.   

When the air flows around the truck and trailers' body work, it creates a LOW pressure (IE vacuum) in the bed and underneath the goose area. That is why things are lifted out of the bed, or lifted enough that they can then slide to the rear. The "bubble" the experts spoke about, is a result of the trapped air being drawn into the bed.

Bernouli's principle is also the reason a million pound machine can fly.

Not to sound argumentative, BUT are the things lost out of the back of the pickup truck Blown out or are they Sucked out??    Bernoulli's principal would apply if the objects were sucked out ie a low pressure area.     Simple wind velocity would blow the items out.  This would be caused by the speed of the air over the cab and being buffeted or trapped and deflected by the front of the trailer.  This should cause high turbulence??     Just saying in observation of the rubber mat and the ramps in the bed of my truck act as though they are being buffeted by strong swirling gusts.