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  1. #1
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    Enkeiavalanche's Avatar
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    Default Pickup Truck Tailgate Up Or Down Aerodynamics Tested

    Wind tunnel testing by GM proves which method provides better fuel economy

    By TTM Staff

    There's been a long debate over fuel economy performance when a pickup truck's tailgate is up or down. Most truck owners agree that when the tailgate is down, it provides better airflow over the cab and reduces drag in the bed. But a recent wind tunnel test by GMC, performend on the new 2014 Sierra, tells a different tail.
    According to Diane Block, GMCs aerodynamic engineer, leaving the tailgate up is more efficient than having it in the lowered position. GMC tested all aspects of the truck's aerodynamics for improved airflow and overall fuel efficiency. According to Block, the when driving with the tailgate up, the air flowing over the cab tends to fall down and then push forward against the truck, amost propelling it from behind. When the tailgate is lowered, the air escapes and doesn't add any benefit to the vehicle at all. In general, GM found that trucks get better fuel economy with the tailgate up.
    While it may seem like GMC's testing shows an open bed with a closed tailgate, there are other facts to consider. The major one that has been overlooked, is that the testing was done in a 750-foot long wind tunnel with a fan that generates winds up to 138 mph. GM did not disclose or provide any indication of the wind speeds in which the 2014 GMC Sierra was tested.
    Bloch also advised against nets covering the back of the truck. "Replacing the tailgate with an aftermarket net is worse than having no tailgate at all," Bloch said in a statement. "Imagine dragging a solid object and a fishing net through water. The net is going to require more muscle."
    Readers Respond



    Check out the new Rear bumper.....Avalanche style...
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  2. #2

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    @Enkeiavalanche,
    Road & Track reported on Canadians doing wind tunnel testing on a F-150 in 2004 (http://www.roadandtrack.com/go/news/...gate-up-mostly) and concluded the same thing. No wonder GM hasn't paid back its government loan(s) ... it keeps spending money to reinvent the proverbial wheel.

    Not sure what I think about the bumper, by the way...

  3. #3
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    We figured this out in the diesel world a while ago...by watching the smoke from guys' stacks. One with the gate up simply floated across the bed and down, one with the tail gate down got swirled around in the bed. I want to remember a couple guys tried to see if it affected their race numbers, but I can't remember the outcome.

    One thing to add to this discussion is the tailgate cap. My experience is with Dodges, but the GMs are similar. If you look at a 1500, they have a cap that is almost a small spoiler, while the HDs don't have that feature. If you watch the smoke pass along a bed (gate up) without the cap, it rushes by the tailgate, then slams downward onto the bumper. The guys with the 1500-style tail gate cap installed would have the smoke exit smoothly, just past the bumper. It was noted by several Dodge owners (myself included) that, by simply adding the 1500 tailgate cap to a 2500 Cummins-powered truck, you could see a noticeable/measureable 0.5 mpg increase in your highway mileage. Point being, the CAFE standards don't apply to the HDs; and therefore don't get all the added mileage maximizing tidbits.

  4. #4

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    Mythbusters also came to the same conclusion. They even used a mesh tailgate and that gave them very slim but better mph numbers over a tailgate.
    2002 Chevy Silverado & 1995 Ford Bronco

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    -Erik



  5. #5

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    The tailgate up results in better MPGs. This has been a fact for years.

    @SurrealOne, from what I've read, GM has satisfied the loan(s) terms. Can we say the same about the 700 billion that we gave the banks? Was I happy when the government loaned GM 49 billion, no. GM does appear to have made a huge turn around since 2009.

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  6. #6

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    Quote Originally Posted by stchman View Post
    The tailgate up results in better MPGs. This has been a fact for years.

    @SurrealOne, from what I've read, GM has satisfied the loan(s) terms. Can we say the same about the 700 billion that we gave the banks? Was I happy when the government loaned GM 49 billion, no. GM does appear to have made a huge turn around since 2009.
    I used parens in "loan(s)" because I was speaking of the 'stock', 'loan', and 'escrow' portions of the bailout which, in total, constituted the 50 billion dollar bailout which is, effectively a taxpayer-based loan/investment in GM. Only the 'loan' portion of it has been repaid, so far as I'm aware. And GM apparently paid it back with cash from the escrow portion. Given this, they're gaming the system every bit as much as the banks (don't get me going on THAT).

    This is probably worth 1.5 mins of your time...



    And this seems to support it:
    http://www.politifact.com/truth-o-me...-says-auto-co/


    Back on topic -- I wonder what a wind tunnel test with a tonneau cover would look like. I'd especially like to see a hard versus soft cover test in addition to comparison with no cover. I say this because the pressure zone might change based on materail type ... as opposed to complete lack of it -- and that would potentially change the shape of the air flow.

  7. #7

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    Stay on topic.

    Steve
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  8. #8

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    I don't like those rear fender flairs or wheel liners on that truck. I think that it looks cheap

    1995 Silverado 4x4
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  9. #9

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    Quote Originally Posted by Pikey View Post
    I don't like those rear fender flairs or wheel liners on that truck. I think that it looks cheap
    I agree, I really don't like that plastic around the wheel wells either.

  10. #10

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    I would also like to see them move the smoke stream down. I want to see why there is a "mud flap" in front of the rear wheel and not behind. It has to be because of some effect on air flow around the tire.

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