Tailgate: Up or Down Saves Fuel?

[johnnymk]

From a Mythbusters Episode Last Month:

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.

http://kwc.org/blog/archives/resources/2005/mb/tailgate.up.s.jpg

Conclusion: Myth busted

[MikeD]

Makes it much quicker and easier to park your truck, though. ;)

I drive with my tailgate down pretty frequently. With the proper bedliner or mat, you can even keep stuff in the back of your truck without them falling out.

Someone once told me it was a violation of the law to drive with it down, but I'm not buying it. :thumbdown

[Cheesypuff]

lol...when I saw the title of the thread...I knew I was gonna quote the mythbusters I saw. Anywho...looks like you already did it. The water tunel with the oats they made was pretty sweet. I LOVE MYTH BUSTERS!!!

[johnnymk]

I LOVE MYTH BUSTERS!!!

Not if you had a hair lip :king:


Man, that was bad!

[gear02]

yeah I love mythbusters too. But why did you post this? Did you drive with the tailgate down?

[johnnymk]

yeah I love mythbusters too. But why did you post this? Did you drive with the tailgate down?

No. I always thought if I ever did, that it would save fuel.

[mechmike0034]

Ahh, physics and aerodynamics...

You can break traffic laws and get away with it (sometimes), but there's no breaking the laws of physics. There's also no such thing as a free lunch.

[BigJon]

Mythbusters is teh roxorz.

Don't forget the one about fuel efficiency and driving with windows down or air conditioner on. They did this twice. Once normally, and once revisited. When they revisited it, they found that it's more efficient to drive with windows down UNDER like 45mph. OVER 45mph it's more efficient to drive with windows up and air on.

I love that show.

[Merlin]

Someone once told me it was a violation of the law to drive with it down, but I'm not buying it. :thumbdown
Yes, because there is now a portion of your car extending beyong the bumper. That then becomes a safety issue unless you red flag it.

[Cheesypuff]

Not if you had a hair lip :king:


Man, that was bad!


what is a hair lip if I may ask...????????

[Sirrich3]

Yes, because there is now a portion of your car extending beyong the bumper. That then becomes a safety issue unless you red flag it.

I heard the same thing too

[Markel]

A long time ago I read that the "gator nets" in place of the tailgate made mileage worse, so this comes as no surprise.

[Airencracken]

<---Aero Major says :duh:

A tonnaue cover will be better aerodynamically but adds weight.

[InfiniteNothing]

Conclusion: Myth busted

I've got to throw a flag on the play. Their measurement device is not sensitive enough to pick up the difference. Engine variations and speed variations are more than enough to account for the difference.

My understanding of fluids is that going slow, the tailgate should act as a parachute of a sort giving it worse milage. Driving fast (tailgate down) you'll get that circular air pattern covering the entire bed and you won't have the sucky vortex you see after tailgate when it's up.

In conclusion I still think down will do better all things being equal. The question is though, by how much and I'm far to lazy to figure that out unpaid.

[blueindian]

I've got to throw a flag on the play. Their measurement device is not sensitive enough to pick up the difference. Engine variations and speed variations are more than enough to account for the difference.

My understanding of fluids is that going slow, the tailgate should act as a parachute of a sort giving it worse milage. Driving fast (tailgate down) you'll get that circular air pattern covering the entire bed and you won't have the sucky vortex you see after tailgate when it's up.

In conclusion I still think down will do better all things being equal. The question is though, by how much and I'm far to lazy to figure that out unpaid.

they were talking about this on the radio the other day and had some auto designers on. according to them, new trucks are designed for better gas mileage with the tailgate up. they set it up so that the airflow forms a "virtual tonneau cover."

[DarkFury]

<---Aero Major says :duh:

A tonnaue cover will be better aerodynamically but adds weight.
Not if it is a soft cover like mine...

The weight is negligible. :D

[johnnymk]

they were talking about this on the radio the other day and had some auto designers on. according to them, new trucks are designed for better gas mileage with the tailgate up. they set it up so that the airflow forms a "virtual tonneau cover."

I would like to believe that the designers set out to do that. In reality, it just so happened to behave that way.

The design of pickup beds hasn't changed in fifty years. Besides, there are narrow beds (stepsides) and wide beds, 5 foot, 6-1/2 foot and 8 foot long beds.

They are taking credit where none is due.

[blueindian]

I would like to believe that the designers set out to do that. In reality, it just so happened to behave that way.

The design of pickup beds hasn't changed in fifty years. Besides, there are narrow beds (stepsides) and wide beds, 5 foot, 6-1/2 foot and 8 foot long beds.

They are taking credit where none is due.

maybe, but i would imagine it has more to do with the shape of the vehicle itself versus the bed in particular.

[Merlin]

For truck designers the rule of thumb for the bed is that it should be able to accomodate a standard sheet of plywood.