John H - I've read a lot of that. And any company that sell tonneau's (or any product) and then talks about things like fuel mileage advantages ... well I throw it out. I'm not saying all your links are some company doing that, obviously they are not. And I'm not saying that tonneau covers don't save fuel. I actually believe they are better than a open bed with tailgate up. But I only consider data. I am a design engineer, and have learned all too well that I can get my **** in some deep you know what, just going by what I hear people say. I've got to have data, and I've got to be able to understand that data.

The university study of drag on the Dodge Ram is data, and is what I tend to believe. I have sent an email to the professor who had posted that data to see if he can provide it to me. I will post it if he sends it to me.

As for tailgate up / tailgate down. I prefer with the tailgate up. When I look at other trucks, they look way cooler with the tailgate up going down the road. Any fuel savings is negligible in my opinion and would only be seen on interstate runs. And all the issues with tailgates flying off, yada yada yada.

As for the structure, Dodge tailgates only sit on the bottom hangers, no structure advantage there. As for the lock area, my tailgate can actually move maybe a quarter inch side to side up there. So the rear walls of the bed can still cantilever from the bottom (vibration during driving), which the steel and welds from the bottom rear of the bed could still fatigue and crack over time. The tailgate would help the bed rails stay upright if the welds broke. But the trucks I've seen with no tailgate and bed structure problems have always been junkers that look like they took the tailgate off because it was likely damage beyond the point it would lock back in place or fit between the bedwalls that have been bashed in from heavy use on construction sites. So, I'm not saying I know (no data), but I'm saying that I'm not sure taking the tailgate off actually leads to the rear bed rails sagging in where the tailgate was. Maybe, but not sure. Oh well, a different discussion I guess.

- JyRO

 

Going strictly from memory, wasn't the difference negligible either way? Something like 2% or less?

 

But from the university study I read, if I remember correctly, the results derived were given in drag coefficient. Now if you go and mathematically solve for the difference in drag coefficients, the differences may likely be very small. But using each drag coefficient in a dynamic situation ... where the FORCE due to drag is a squared relationship to ground speed, those small differences in drag coefficient start making a more than negligible difference at around 65 to 70 mph. So yes your right, but not painting the whole picture, at least as I understand it. Was that as clear as mud?

- JyRO

 

Maybe I missed it. I thought they gave their results in a mpg form, and the difference was less than 1mpg in nearly every case. Like I said, I was going from memory.

 

Perphaps not as quantitative as some would like, but. . .

http://www.awtrucks.com/tailgateup.htm (probably need to cut and paste)

Dear Tom and Ray:

I read your recent article in which a reader (also named Kathy) asked whether a pickup truck gets better gas mileage with the tailgate up or down.

You guys said "down.".

My husband and I made a very large bet about the very same issue. I work for GM as an engineering intern and had a chance a couple of weeks ago to tour the design facilities in Detroit. When I got to the wind tunnel building, I asked the engineers this same question.

They laughed and demonstrated that trucks are designed so the airflow creates the least amount of drag when it flows off the roof and past the tailgate in the upright position.

They said that leaving the tailgate down would actually decrease a truck's fuel mileage. So guess who won the bet, guys?

Kathy

RAY: We know who won the bet, Kathy. because we received letters about this from engineers scattered throughout the automotive industry.

TOM: Here's one that offers a more technical explanation for you (still) nonbelievers.



Dear Tom and Ray:

I'm an aerodynamics engineer. When I was in the U.S. Air Force a few years back, I worked with folks from the Lockheed low-speed wind tunnel.

In the 1970s aircraft production went into a slump, and Lockheed started looking.for other customers for its wind-tunnel services.

Prime candidates were automakers, and Lockheed was successful in convincing Ford, among others, that the wind tunnel wouId help them reduce drag and wind noise on their vehicles.

Needless to say, in the past 15 to 20 years, Lockheed has learned a lot about car and truck aerodynamics.

Anyway, they actually performed drag tests on pickups with the tailgate both up and down, and found that drag was actually LOWER with the tailgate CLOSED!

This ran counter to their intuition (and yours). The reason is that a closed tailgate sets up a large "bubble" of stagnant air that slowly circulates around the bed of the truck (we aero types call this a ("separated bubble"). When air approaches the truck, it "sees" the bubble as part of the truck. So to the air, the truck looks like it has a nice, flat covering over the bed, and the air doesn't "slam" into the vertical tailgate.

If the tailgate is open, or replaced by one of those "air gate" nets, however, that nice, separate bubble in the truck bed does not form (it "bursts").

Then the air approaching the truck "sees" a truck with a flat bed on the back of a tall cab. This is a very nonaerodynamc shape with a very LARGE drag.

So, believe it or not, it's best for gas mileage to keep the tailgate CLOSED. Hope this information is helpful.

Ed Fitzgerald
Research Assistant
Department of Aero/Mechanical Engineering
University of Notre Dame

TOM: Sounds pretty convincing, Ed. Thanks. We also heard from none other than former GM President Bob Stempel, who wrote to say that aerodynamically it doesn't make that much difference. But, he says, a pickup truck is structurally much SAFER with the tailgate up.

RAY: So for that reason alone, we suggest you throw away those tailgate nets, folks. And as your flight attendant might say, please return your tailgate to the upright and locked position.

 

talk talk talk, bla bla bla ... show me the data. I could care less about who worked where and said bla bla bla about it. I've read all that before. It means nothing. Its still opinions, conjecture, and propaganda. DATA wins. Until it is proved by data, I don't believe the wind tunnel 'talk.'

I'm still trying to get the university study data.

- JyRO

 

I don't have an opinion either way on what's better, tailgate up or down. I'm enjoying the banter however...
Let's through another variable into your discussion... What happens to mpg when your tailgate is up (or down) while you're pulling a 5th wheel? That's something to chew on eh? I don't have an opinion either way, but what's yours?

 

I hope its coming off as friendly banter, because I'm no expert. Just a know it all.

As for towing, tough one ... but I'm guessing tailgate up. Anything that might tend to influence the air up and over the trailer I'm guessing would be better.

- JyRO

 

I guess, whether it saves gas or not, I'm driving with my tailgate up. I don't like the idea of a loose tailgate banging around. I don't care for the way it looks either. Let me know when you get that data, please post it or a link. I'd be interested in seeing it also out of shear curiousity. FYI I'm a test engineer for a company that makes control valves, so I also live with test data. I have just gotten tired for looking for the numbers on the internet, and have pretty much come to the conclusion that the only person who really knows the answer is the guy that did the test.

 

 

OK, BLAH BLAH - - you should get your facts straight before you poo poo everyone and blah blah.

http://truck-bed-covers.com/Tonneau_..._save_fuel.htm

http://www.roadandtrack.com/article....&page_number=1

http://www.findarticles.com/p/articl...05/ai_n9439015

 

I can't open the last one. I agree that *I think* that a tonneau cover will help with fuel economy.

"They also performed Computational Flow Dynamics analyses of two simplified pickup shapes." Computational, meaning not measured. I don't put a lot of stock in that. If you guys want to believe its better with the tailgate up, that's fine. I'm not saying its better down. I'm saying that *from what I remember from the data*, it was better with it laid down. I like it better up anyway.

If I get that data, I'll post it.

 

Yep, tail gate up. Don't know about with a trailer. Have some leveling boards in the bed in front of the hitch and they used to flip over with the Supercab ford. They don't flip with the QC. Same trailer different hitch but the distance to the cab is the same. Don't get any milage towing a brick.

Phil

 

To carry it further, the tests showed that webbed and slotted tailgates also got lower mpg.

 

One more point on the curve.

The question was asked "what about when towing a 5th wheel?"

Last year, I was driving from New Orleans to Houston, pulling my 5th wheel, with an empty bed. At one of the fuel stops, for some reason I needed to take two big rubbermaid plastic boxes out of the 5th wheel, and not having anywhere else to put them, I put them in the bed of the truck. These boxes measured somewhere about 3' X 2' X 18" each, they each had lids on, and they were empty.
I set off down the road, and through the rear window of the truck, I could see they were moving around quite a bit. Then at about the time I got the rig up to about 65 mph, I hit a bump. One of the boxes flew up out of the bed, and stuck to the underside of the overhang on the 5th wheel, over the bed. It stayed there until I dropped back down to about 45 mph.

I guess there's a low pressure area in that region huh?

I wonder if taking the tail gate off while towing would be better.

John