From Cartalk


http://www.cartalk.com/content/colum...ctober/05.html


ar 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 the auto makers, and Lockheed was successful in convincing Ford, among others, that the wind tunnel would help them reduce drag and wind noise on their vehicles. Needless to say, in the past 15-20 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 nonaerodynamic 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, Dept. of Aero/Mechanical Engineering, U. of Notre Dame

Tom: Sounds pretty convincing, Ed. Thanks. We also heard from none other than Bob Stempel, the former GM president, who wrote us 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.


From Mythbusters.

http://kwc.org/mythbusters/2005/11/e...s_cures_f.html

Tailgate: Up or down?
Myth: Driving your pickup with the tailgate down gives you better fuel efficiency than with the tailgate up.

Test Setup
Adam and Jamie were each given identical, new model pickup trucks. They both had equal mileage, same tire pressure, and 30 gallons of gas. Jamie drove with the tailgate up and Adam the tailgate down.

The rules:

They have to maintain the speed limit
All acceleration must be done by cruise control
No drafting
Windows up, A/C must be exactly the same in both cars
The Test
After 300 miles there didn't appear to be much difference in the gas consumed, but after 500 miles Adam (tailgate down) ran out of gas. Jamie made it another 30 miles before he ran out of gas. This result was the exact opposite of the myth.

Water tunnel visualization
According to the experts, a circular pillow of air forms behidn the cab of the truck when the tailgate is up. This "separated bubble"/"locked vortex flow" keep the faster moving air from contacting the truck, and thus reduces drag. With the tailgate down, the bubble breaks down and is no longer able to keep the fast moving air out, increasing drag.

In their scale model with the water tunnel, they were able to see that the increased drag. With the tailgate down, the particles in the water were dropping down and hitting the tailgate.

mythbusted
~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Tailgate Tilt

Attached is an article printed in the "Click and Clack" automotive column of the Antelope Valley Press in California. I think most pickup owners would like to have this information:

"I read your recent article in which a reader asked whether a pickup truck gets better gas mileage with the tailgate up or down. You guys said 'down.' I work for GM as an engineering intern and recently toured a design facility in Detroit. I asked the engineers in the windtunnel building this same question, and they demonstrated that pickup 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 leaving the tailgate down would actually decrease a truck's fuel mileage."

Another letter offered more technical explanation with the same result-that drag was actually lower with the tailgate closed. The reason is that a closed tailgate sets up a large bubble of stagnant air that circulates around the bed of the truck. 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 bubble in the truck bed does not form. Then the air approaching the truck "sees" a truck with a flat bed on the back of a tall cab. This is a very non-aerodynamic shape with a very large drag.

In addition, a pickup truck is structurally much safer with the tailgate up.

EUGENE COWART

LANCASTER, CALIFORNIA

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~


Just too many Tailgate UP/closed is best articles to dispute it.

http://amos.indiana.edu/library/scripts/tailgate.html

http://ask.yahoo.com/ask/20050427.html

http://blog.kevindonahue.com/archive...s_mileage_.php


No confirmed evidence.

http://findarticles.com/p/articles/m...08/ai_n8808802

http://www.tulsaworld.com/ActionLine...A11_Nosol27233

Taligate better off REMOVED.

http://www.usc.edu/CSSF/Current/Projects/S0106.pdf

http://www.gainesvilletimes.com/news...ws/59878.shtml



So FTEer's

Let's get in on the google search for this topic.

Do your own Tailgate up/Tailgate down ....testing with hopefully identical conditions to compare the results.


1) Same road,
2) Same Truck/Engine operating conditions on I suggest the same day.
3) Re-filling your tank to the exact same condition before the drive
4) No alterations except tailgate removal

 

Just happened to be looking at aerodynamics myself.

http://www.sema.org/Main/SemaOrgHome.aspx?ID=52546
basically says toneau covers are best.

Also flat smooth hub caps, air dams, underbody panels, etc all are also supposed to help.
Aero pickup

I'm going to test the toneau on mine. I'll let you know on that score.

 

I wonder if bed lengths make any difference in the wind drag?

 

I doubt the bed length makes much of a difference. The basic idea is that when driving, the air goes over the truck and past the bed. The air can't go into the bed (to hit the tailgate) because the air already in the bed has nowhere to go, thus the bed acting like a fully enclosed space, and the air just goes over the bed and over the tailgate and past the truck. Hopefully I sorta made my point... I like physics but I a bit drunk...

 

My truck's too old and boxy to be even considered aerodynamic.

 

I feel your pain. But I hope to prove you wrong. I'm toying with the idea of aero mods IF I get positive results with the bed cover.

 

I have always heard newer trucks cabs are shaped to direct the airflow over the tailgate.That thought never even crossed fords mind when they made our trucks.
I can see how the tailgate would add more support when closed.

This is one of those topics like which is built better ford or chevy.It could go on forever.


Tim

 

what do you mean? the answer is Ford.

 

No question, have you seen the build quality of even new(2000+) chevys? Panels misaligned, doors hung crooked, etc. I'm very disappointed in some of the Ford's recent screwups(6.0 fuel system, cruise control fires, etc) but I firmly believe their build quality is still higher than Chevy.

But back to the original now.... test it! Give mil1ion some data. I'll have my first control sample sunday or monday.

 

If it made a difference to have the tailgate down or have a cover the trucks would be produced with "options" to take advantage of those things and the configuration tested for CAFE would reflect those options.

I have seen too many documented and well done scientific tests that have shown that tailgate up is best. The only evidence I have seen to the contrary has been anecdotal or hearsay. None of the links in post #1 references physical scientific tests.

If you have the bucks you can buy verifiable research results here:
http://www.sae.org/servlets/index

 

I remember reading about a test that Archer Brothers racing did a few years back. They were in the SCCA Sport Truck Road Racing series. Rangers/S10s/etc. Anyhoo, they performed tests to see if having the tailgate on and up or removed made any difference to lap times. It didn't make any noticable difference. This was performed on Jeep Comanches that were race prepped. My guess is that any advantage from removing the tailgate, ie. weight savings, was offset by the increased drag. And the Comanche was more of a brick than our F series.

 

Great link. I was surprised to see that they go as far back as the 1980 F150.

Why is the CAFE argument not aplicable on older trucks?
First enacted in '75, no numbers on pickups until '79, no full results until '84.
"The near-term goal was to double new car fuel economy by model year 1985." and there's why nothing significant until '84.
"Passenger car standards were established for MY 1978 (18 mpg)" first year '78 for cars.
"Congress did not specify a target for the improvement of light truck fuel economy. Instead, it provided that light truck standards be set at the maximum feasible level for model year 1979 and each model year thereafter."
results: '84-'04 2wd full size pickup mileage increased a whole 1.5mpg. wow.
CAFE site
http://www.nhtsa.dot.gov/portal/site...7529cdba046a0/

That option argument sounds good, but I think you missed a key point. Ford is a business whose primary goal is profit. They will provide options that they think will be profitable, regardless of the efficiency or lack of same.

For example, how many diesel owners on this site have gotten more power and economy by deleting their muffler and adding a big exhaust system? Better gas mileage and more power? Should be a no brainer for Ford. But they don't offer it do they? Apparently they think the cost of providing the muffler delete as another option, which could be millions in brochure changes alone, outweighs the profit possible from it.

As a further example, look at the aftermarket industry as a whole. There are hundreads of companies profitting on options Ford does not provide. Obviously there's money to be made, many of these companies even have better stock ratings than Ford, but Ford doesn't believe they can profit from offering any of those options.

Basically every aspect of a given car is designed by an engineer, artist, or accountant. That specific aspect's primary purpose is boosting sales, not maximium power, economy or durability. That is why there is often room for improvement.

 

You are right on the money derherr65!

 

The OEM's in general will jump on anything that will provide 0.1MPG difference nowadays. We definitely would see the "tailgateless option" or recommendations for "tailgate down" from at least one of the OEM's if it worked. Regardless of marketing or stylists. They are constrained by emissions and noise regulations as in diesel exhaust systems but that does not apply to a tailgate or cover.

 

It just doesn'tmake sense that it works that way. If they were that determined then we wouldn't see any wiper blades above the hood line, big flat surfaced sideview mirrors, etc...

Youre just fooling yourself thinking that at least Ford is worrying about every .1 mpg in a pickup truck used for hauling or whatever work it will be used for. It is more important to Ford (who is laying off people like crazy) to get a happy medium in gas mileage, have an attractive useable and durable pickup that sells good and not cost more than x number of dollars to produce.

I believe that Ford trucks being produced today would appear much different than they do if every .1 mpg was genuinely a top priority.