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I thought this was quite interesting.

Theres a popular myth with pick-up trucks that driving with your tailgate down gives better gas mile-age and makes the truck more aero-dynamic.
The mythbusters did a test to see if this was true or not and here are the results:
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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
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And the fact that every truck has different bodylines, and will have different airflow over/around it.

You'd have to run that test with every configuration of each model of pickup. What may hold true for an extended cab Silverado, for example, could be the opposite of what holds true for a crew cab Colorado.
 

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Mythbusters is a great show. If Adam and Jamie tell me the world is flat... Then the world is flat. After all, I have answered my cell phone at the gas station many times, No one every got burned to a crisp. And when being shot at, a quick dip in the pool has always saved my life.

:D
 

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You could probably drive a Colorado with the AC cranked, windows down and tailgate down and still get better MPG than most other trucks. :D
 

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ZQ8Dude said:
i disagree, aerodynamics of each truck may vary the results, but overall it will be the same outcome. Tailgate up is more efficient.
You're entitled to your wrong opinion. I can tell you that the regular cab S-10, for example, doesn't follow that.

And yes, that was actual lab testing. Not two people in similiar trucks. Not the same truck on different days. Not two people in different lanes on the same road.
 

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And if you wax the inside of the tailgate itself - your aerodynamic advantage will increase even more....with the tailgate up, of course! :D
 

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From my experience with this truck the difference is negligible. My best ever mileage with the gate on was 23.5, my best ever with the tailgate removed was 23.75. Not enough of a difference to bother. I did notice people seemed to stop a lot closer behind me at stop lights without the gate on. I would guess it was because they were looking at the front of the bed, there wasn't as much surface area there for them to gauge their distance from me. There's no way I would drive around with the tailgate just laying down, it would drive me nuts.
 

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naskie18 said:
You're entitled to your wrong opinion. I can tell you that the regular cab S-10, for example, doesn't follow that.

And yes, that was actual lab testing. Not two people in similiar trucks. Not the same truck on different days. Not two people in different lanes on the same road.
What are the circumstances of this testing? Wind tunnel or water tunnel drag coefficient measurement or some sort of test that actually ran the engine? Did it use an actual vehicle or a model? I just ask because that result does not coincide with the basic principles of low speed aerodynamics.

The last half of a body in a fluid flow is much more critical in drag than the first half and especially the angle of the rear of the body. If the body narrows too quickly, you tend to get flow seperation which increases drag by a large margin. The area of recirculation produced by the tailgate effectively lowers the rate at which the truck body narrows because it acts like part of the truck reducing or preventing this flow seperation. Anything more than a 20 degree slope off of flow direction starts to give separation and without that area of recirculation you're looking at 90 degrees at the back of the cab.
 

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ZQ8Dude said:
naskie18 said:
ZQ8Dude said:
i disagree, aerodynamics of each truck may vary the results, but overall it will be the same outcome. Tailgate up is more efficient.
You're entitled to your wrong opinion. I can tell you that the regular cab S-10, for example, doesn't follow that.

And yes, that was actual lab testing. Not two people in similiar trucks. Not the same truck on different days. Not two people in different lanes on the same road.
really? is that why that S10 that was build for a landspeed run DIDNT have the tailgate down and used a partial tonneau cover?

obviously it wasnt aerodynamically effiecient.
I think that is because of the high speeds is why they neeed the tailgate and partial tonneau.

But then again I just leave mine up because its not a big deal.
 

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nova said:
What are the circumstances of this testing? Wind tunnel or water tunnel drag coefficient measurement or some sort of test that actually ran the engine? Did it use an actual vehicle or a model? I just ask because that result does not coincide with the basic principles of low speed aerodynamics.

The last half of a body in a fluid flow is much more critical in drag than the first half and especially the angle of the rear of the body. If the body narrows too quickly, you tend to get flow seperation which increases drag by a large margin. The area of recirculation produced by the tailgate effectively lowers the rate at which the truck body narrows because it acts like part of the truck reducing or preventing this flow seperation. Anything more than a 20 degree slope off of flow direction starts to give separation and without that area of recirculation you're looking at 90 degrees at the back of the cab.

I found out a long time ago just about no one on this board or just about any forum for that matter cares about the scientific reasons that cause phenoma to happen. There is an aritcle floating around somwhere in which fluid mechanics engineers at some university in New England designed a truck cap the cut the drag coefficient of a Dodge Ram by over 30%, dropping the Cd to that of a corvette.
 

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i think the really simple 'no ****' answer is "if the truck ran more efficiently with the tailgate down it would come that way" or your tailgate would have holes in it or lowering your tailgate would be recommended in your truck manual....which it's not. in this day and age of high gas prices i think it would be a safe bet that vehicle manufacturers would advertise every once of fuel economy they can squeeze out of a truck, and if they could squeeze another mile or two out of a tank of gas with the tailgate down they would let you know....


plus those guys in the wind tunnel with all of the high falootan doo hickeys and thinga-majigs proved it...
 
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