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mileage better w/ tailgate UP????

Discussion in 'The Garage' started by tiger9297, Aug 19, 2006.

  1. tiger9297

    tiger9297 1/2 ton status

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    Just saw on Mythbusters where they disproved the notion that driving with a truck tailgate down improves gas mileage. They showed how with the tailgate up there is a rolling "ball" of air right behind the cab that deflects the oncoming air up and over the rear of the truck bed. With the tailgate down the oncoming air falls rapidly right over the cab into the bed of the truck creating LOTS of drag. Who'd a thunk it??
     
  2. big dan

    big dan 1/2 ton status Premium Member

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    yeah I just watched the same episode. Ive heard of that before but never knew which one was true. guess Ill keep on driving with the tailgate up.:D
     
  3. fabjunkie

    fabjunkie 1/2 ton status

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    I saw that one too. I wonder if it depends on the shape of the truck, cause on my Toyota, I actually got another mile or two per gallon with it down. I had a toolbox on it though, so I'm sure that redirected the air too. I'm not going to say that they are completely wrong, but I have to go off of experience too.
     
  4. muddybuddy

    muddybuddy 3/4 ton status

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    i wonder how it would effect a lifted truck as well.
     
  5. Metrodps

    Metrodps Strange but nice guy Premium Member GMOTM Winner Author

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    They also did a test with 2 exact vehicles, motor, body type and weight. Drained all the gas out and I believe it was 5 gals of gas they added to them one ran with AC on and the other with windows down. Had some gauges on it and used cruise control drove around a track till trucks ran out of fuel and at highway speeds the AC was better mpg than windows down. Go figure on that.

    Got to love them shows they do some gnarly things on them.
     
  6. diesel4me

    diesel4me 1 ton status Premium Member

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    yeah....

    You mean like the 4 foot "WOOFER" they built inside the Mercedes,and used a crankshaft hooked to the driveshaft to run it????....:bow: :haha: ....

    I'd have rather heard a REAL electric speaker that big...something that huge must make some DEEEP bass notes!..:crazy:
     
  7. ssped

    ssped 1/2 ton status GMOTM Winner

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    I have a hard time believing that one. I drove to san jose on a tank of gas with the tail gate up. I filled up there and drove home with the tail gate home. I made it home and drove to work twice and home once befor having to refill. it was significantly better with the tail gate down.
     
  8. Metrodps

    Metrodps Strange but nice guy Premium Member GMOTM Winner Author

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    I won't post the whole long article but if you want to read about this http://community.discovery.com/eve/forums/a/tpc/f/9401967776/m/9241965418

    Using a variety of popular make full-size trucks and a 75-mph wind speed, the vehicles were tested in a wind tunnel with the tailgate up, down and removed.
    One had a mesh gate made of duct tape. One had an adjustable tonneau cover that could cover various portions of the truck bed.
    The wind tunnel balance-measured all six components of aerodynamic force and moment. Drag coefficients (Cd) for the trucks were normalized so the focus could be kept on box aerodynamics. All the measurements showed the same basic results-a small increase in drag with the tailgate down and a larger increase with the tailgate off. These measurements were made with the vehicle directly aligned with the stream of air.

    Another interesting result was that lowering or removing the tailgate caused a larger increase in drag on 61/2-ft. boxes than on 8-ft. boxes. Installing the mesh gate was shown to yield the worst results of all. In one example, the Cd increased from .48 to .53 with the mesh gate in place of the solid tailgate.

    What does work is a tonneau cover. Interestingly, a partial 20-in. cover placed nearest to the tailgate did 88% as well as a full tonneau cover. On an 8-ft. box, the estimate is that the improvement in fuel economy with a full tonneau could be 1.35 mpg, or a saving of 243 gallons over 100,000 miles of driving. Conversely, the same vehicle without the cover and with a mesh tailgate got 1.28 mpg worse mileage than a vehicle with the tailgate up and no cover.

    The wind tunnel and road tests answer the question of whether removing the tailgate is a good idea. What it doesn't answer is why. For this, Cooper and his group used CFD software to model the flow of air in and around the box of a pickup truck. They found that with the tailgate up, a bubble of air forms in the truck bed that allows air flowing from the front to the rear of truck to ride up and over the vehicle. When the tailgate is removed, there's a stronger downwash that forces some of the air to flow over the rear bumper. This stronger downwash behind the cab is what's responsible for the increased aerodynamic drag.

    Among the conclusions in his paper, Cooper points out that for an average pickup, the fuel saved by leaving the tailgate up is likely to be on the order of 122 gallons over 100,000 miles of freeway driving. On an individual basis, this is not a lot of fuel or a lot of money. However, when you apply this to the millions of vehicles out there, this really is large.
    And from a safety standpoint, having the tailgate down makes it and not the bumper the first point of contact in a rear-end collision. Based upon the data-and the cost of gasoline-having a full or partial tonneau might be worth the investment.
     
  9. k204dr

    k204dr 1/2 ton status

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    so, does a camper shell help or hurt? anyone ever tested that?
     
  10. muddybuddy

    muddybuddy 3/4 ton status

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    what bout with taking the hardtop off the k5 vs. keeping it on? ive got a tonneau cover type soft top as well, i guess that would be the best.
     
  11. Chevy305

    Chevy305 6 Lug 14bsf Status Premium Member GMOTM Winner

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    That is why when ever you open the back window on the cab you always get that air blowing in real strong. Cooling on a hot day.


    Speaking of cooling I don't think the windows vs. AC on our trucks really counts because we are all just driving large bricks anyway, I don't think a little more drag from the windows will noticably hurt milage. Also why is it that when the AC is on on any car or truck the engine feels gutless? But when the windows are down I can't tell a difference in power to when the windows are up.

    I guess I will keep driving with the windows down and the music blasting. :D
    (I kinda have to because I have no AC) :haha:
     
  12. Tabb

    Tabb ROLL TIDE! AGAIN! GMOTM Winner

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    What if you've got one of those cheezee-lookin' cab spoilers on the back of the roof, do you think that would help?
     
  13. Tabb

    Tabb ROLL TIDE! AGAIN! GMOTM Winner

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    I thought we were talking about pick up trucks on this thread. I know what the spoiler on the back of k5 is for. I don't think those are cheezey, In fact I wish mine came with one. What I was talking about was those after market spoilers for Pickup trucks that go across the back edge of a pickup trucks roof. Has anyone ever seen one? I've always thought that would disrupt the areo across the bed.
     
  14. muddybuddy

    muddybuddy 3/4 ton status

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    ive ever seen a k5 with a spoiler? what have i been missing?
     
  15. ntsqd

    ntsqd 1/2 ton status

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    Ever wonder why a water drop is shaped the way it is?
    The vacuum on the back side 'pulls out' that "tail".

    With the tg down you have the full height of the cab + the depth of the bed that the air has to fill in. With the tg up it traps air in the bed and the air coming off the cab roof actually very slightly compresses the air in the bed. Kinda, sorta makes the air in the bed act like a tonneau cover. Which makes the transition from the top of the cab to the rear bumper more gradual since it's two smaller step spread out over some distance rather than one large step.

    The "spoiler" being referenced came on some Subs & I think K5's as an option. It is an air deflector designed to take some of the air coming along right at roof level and push it down the back window.
    Those Lund cab spoilers have entirely the wrong shape to do much good. I fully expect that the hurt more than help.
     
  16. Blue85

    Blue85 Troll Premium Member

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    The real solution is to use a tonneau cover. It eliminates the large cavity of the bed.

    There is a lot of anecdotal evidence that tailgate down or 80's mesh gate improves gas mileage, but how many real tests have those people done with the same load, same road, same wind conditions, etc? Few people are patient enough (several tanks of gas) or scientific enough to really know what things affect their mileage and by how much.
     
  17. muddybuddy

    muddybuddy 3/4 ton status

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    the easiest thing i know to help gas milage is tire pressure.

    so with a k5, would it getter economy with the hardtop on or off?
     
  18. daleearnhardt01

    daleearnhardt01 1/2 ton status

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    Its the same principle why would it be any diffrent? And with a truck as large as yours none of that really applies anyways, lots of air is going under the truck etc..

    You have never seen a Suburban or Blazer with the wing on the back? Its just a little spoiler that helps keep the rear window clean by redirecting airflow..
     
  19. 87BrnRsd

    87BrnRsd 1/2 ton status

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    They used a computer hooked up to the vehicle's (ford explorer) computer to make the calculations in fuel economy. But...if you continued to watch the show, they drained out all the gas again, filled them up with 5 gallons, one with ac on and windows up, and one with windows down/no ac...and drove them around the test track again. This time however they didnt use a computer to measure the mpg, but just drove both around the track until they shut off. The one with the windows down went 40 more laps. They "busted" the myth in favor of windows down gets better mpg...
    -Harrison
     
  20. muddybuddy

    muddybuddy 3/4 ton status

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    nope:doah: never seen them before....not many older chevys around here. everybody drives newer cars.
     

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