will a tonneau cover make a noticable diff in mileage?
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From: Flat Rock, MI
There was some link to some scientific study on here a little bit ago that basically came to the conclusion that you don't get any benefit from a tonneau cover... something about how the air flows w/ an open bed as opposed to a covered one... 
Personally, I leave mine rolled up unless I'm going to be putting luggage in the bed for traveling... I don't notice a difference either way...
Tony
Personally, I leave mine rolled up unless I'm going to be putting luggage in the bed for traveling... I don't notice a difference either way...
Tony
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From: State of Confusion
From all that I have read, Tailgate up or down makes no difference.
As for the Cover, I read a test result that said at VERY high speeds it does make a difference, but, not at normal highway speeds. That test result also said that windows open or closed made more difference than cover on or off.
I have read so many conflicting reports, I don't know what to believe.
As for the Cover, I read a test result that said at VERY high speeds it does make a difference, but, not at normal highway speeds. That test result also said that windows open or closed made more difference than cover on or off.
I have read so many conflicting reports, I don't know what to believe.
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I'm guessing windows up helped the mileage. If anything, I'd think a tonneau cover would hurt mileage cause of the extra weight. Regarding the tailgate up and down, I have read several reports that said you will lose mileage with the tailgate down.
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From: WA state
Some graduate students at the Western New England College measured drag coefficients on a model 2nd gen Ram. Stock with tail gate up was 0.44, with a tonneau cover it was 0.38.
When ground through a bunch of calculations, this would amount to 1 mpg at 70 mph. Of course, if you have hung a bunch of other stuff on your rig that breaks up the air flow, you might get no improvement.
When ground through a bunch of calculations, this would amount to 1 mpg at 70 mph. Of course, if you have hung a bunch of other stuff on your rig that breaks up the air flow, you might get no improvement.
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From: St.Paul , MN
That a pretty good drag coeficient for a big truck. My old 84 Olds Station wagon had about .45 according to the list of cars that was published a while ago. My T-birds Super Coupe had about .30 which was about as good as cars get outside of Vettes and clean sports cars.
If a cover got it down to .38 it would be amazing. That would be in line with most full size cars of today.
If a cover got it down to .38 it would be amazing. That would be in line with most full size cars of today.
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From: Tidewater Virginia
I have no data that shows a tonneau cover improves air flow and reduces fuel consumption. But, it does make sense that anything done to reduce the drag or turbulence would help. I have a hard tonneau on my truck that also has a spray-in liner. In my opinion, this helps quieten the truck. Incidently, my truck gets about 25 MPG on level highways at 60MPH and 20+ MPG local driving.
The draw back to a tonneau cover is that you can't throw your empty beer cans and trash through the sliding window into the bed.
The draw back to a tonneau cover is that you can't throw your empty beer cans and trash through the sliding window into the bed.
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From: Where hell freezes over.
The only thing you will gain by leaving your tailgate down is nice little chips all over it. Seen one at the dealer the other day, the truck was absolutly beautiful until I walked around to the tailgate!!
Ran truck with a tonneau cover and without, no difference in mileage either way.
Ran truck with a tonneau cover and without, no difference in mileage either way.
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Joined: Apr 2003
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From: Cummins Technical Center, IN
AERO 101:
The most important thing in aerodynamics is how the air is allowed to come back together after it's been disturbed.
It's well-known that the most aerodynamic shape is that of a tear-drop. A falling drop of water naturally assumes this shape because it's the "path of least resistance" so to speak.
That teardrop has two end-- the pointy end, and the rounder end. What would happen if you took that rounder and and flattened it a little bit, like you just sliced the nose off? Now you'd have a flat surface on one end and the same tapered pointy end on the other.
Now take this flattened teardrop shape (like you took a wet Q-tip and squished the end) and put it in an airstream. Which would be more aerodynamic- pointy end first, or flat end first?
Flat end first would be. While the flat end doesn't punch as clean a hole in the air, the pointy end behind it lets the air come back together with minimum disturbance.
If you switched it around with the pointy end first, the air would have to come back together in a haphazard way (because it's just a flat trailing edge). Air doesn't like to do this, and it increases drag.
Ever wonder why a flag ripples in the wind? Or why a square-backed big-rig trailer wants to sway? It's because of the aerodynamics.
Sorry to ramble, but here's how this relates to our trucks: We have these huge squared-off trailing edge surfaces-- one is the back of the cab, one is the back to the tailgate. A tonneau cover won't help because you'll still have that squared surface at the back window and the tailgate. Remember, the air is travelling horizontally, so VERTICAL surfaces are the main focus here. All a tonneau cover does is basically make the air think that the floor of the bed is higher-- it won't change the large vertical dusturbances (after tailgate and after cab) in the least.
Putting down the tailgate doesn't help either, because now instead of two vertical dusturbances-- the back of the cab and the tailgate, NOW you have one huge dusturbance at the back of the cab. All you've basically done is moved part of the vertical disturbance area from the back of the bed to the front.
So what would it look like if you had a camper shell that actually helped the aerodyamics? First, it would basically look like a ramp that connected the top back corner of the cab roof to the top of the tailgate. The sides of this shell would be long triangular shapes that sealed this "ramp" to the bedsides. Now, you've eliminated the cab-portion of the vertical disturbance, leaving you only with the tailgate.
The tailgate portion could be addressed either with a bubble-style gate or with something similar.
Airtabs work because they basically trick the air in to thinking that squared-off vertical disturbances are tapered like I described above.
The idea would be to make the truck transition from a squared-off front to a cone-shaped rear with no sharp edges or 90° edges. The closer to that you come, the better you'd be.
Feel free to ask more questions.
jlh
The most important thing in aerodynamics is how the air is allowed to come back together after it's been disturbed.
It's well-known that the most aerodynamic shape is that of a tear-drop. A falling drop of water naturally assumes this shape because it's the "path of least resistance" so to speak.
That teardrop has two end-- the pointy end, and the rounder end. What would happen if you took that rounder and and flattened it a little bit, like you just sliced the nose off? Now you'd have a flat surface on one end and the same tapered pointy end on the other.
Now take this flattened teardrop shape (like you took a wet Q-tip and squished the end) and put it in an airstream. Which would be more aerodynamic- pointy end first, or flat end first?
Flat end first would be. While the flat end doesn't punch as clean a hole in the air, the pointy end behind it lets the air come back together with minimum disturbance.
If you switched it around with the pointy end first, the air would have to come back together in a haphazard way (because it's just a flat trailing edge). Air doesn't like to do this, and it increases drag.
Ever wonder why a flag ripples in the wind? Or why a square-backed big-rig trailer wants to sway? It's because of the aerodynamics.
Sorry to ramble, but here's how this relates to our trucks: We have these huge squared-off trailing edge surfaces-- one is the back of the cab, one is the back to the tailgate. A tonneau cover won't help because you'll still have that squared surface at the back window and the tailgate. Remember, the air is travelling horizontally, so VERTICAL surfaces are the main focus here. All a tonneau cover does is basically make the air think that the floor of the bed is higher-- it won't change the large vertical dusturbances (after tailgate and after cab) in the least.
Putting down the tailgate doesn't help either, because now instead of two vertical dusturbances-- the back of the cab and the tailgate, NOW you have one huge dusturbance at the back of the cab. All you've basically done is moved part of the vertical disturbance area from the back of the bed to the front.
So what would it look like if you had a camper shell that actually helped the aerodyamics? First, it would basically look like a ramp that connected the top back corner of the cab roof to the top of the tailgate. The sides of this shell would be long triangular shapes that sealed this "ramp" to the bedsides. Now, you've eliminated the cab-portion of the vertical disturbance, leaving you only with the tailgate.
The tailgate portion could be addressed either with a bubble-style gate or with something similar.
Airtabs work because they basically trick the air in to thinking that squared-off vertical disturbances are tapered like I described above.
The idea would be to make the truck transition from a squared-off front to a cone-shaped rear with no sharp edges or 90° edges. The closer to that you come, the better you'd be.
Feel free to ask more questions.
jlh