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So here is an idea I have been thinking about for awhile now.

I am sure everyone is familiar with the timeless "tailgate up vs. tailgate down" argument. Most people will tell you that the best mileage comes from having the tailgate in the up and locked position. I respectfully disagree.

There was a study done by GM years ago about the airflow over the cab of a truck. The link for this study no longer works and I am still trying to re-find the information on the internet. The long and short of the study was that as long ago as the sixties GM was aware that a tonneau covering a mere twenty inches of the rear of the bed improved fuel economy. GM did not think enough of this to pursue it in production and so this information gathered dust until someone posted it on the internet and I stumbled across it a little more than a year ago.

What piqued my interest was a passing observation that the length of the cab roof, combined with the bed length, in relation to the air speed over the two had an impact on whether the tailgate should be up or down. For regular cab trucks with a standard bed the tailgate should be up, but for double cabs (or crew cabs) this did not hold true.

For a few years I have nursed the idea that down force on the rear of a pick-up truck (in proper quantity) could improve mileage. This stems in part from the gains purported by the Roadmaster suspension system along with my personal observation of loading the truck down with all kinds of crap for (our used to be) annual trip to the Florida Keys each winter. My theory is that there is some wheel hop (possibly indiscernible) that steals from the overall mileage. Adding weight, applying down force, or soaking up that hop with the Roadmaster suspension would negate that loss.

So, I decided that the whole idea of having the tailgate up to create an air bubble to bounce air over the bed of the truck at highway speeds was invalid. The notion that the resistance (drag) created by the tailgate in the upright position was beneficial just did not sit well with me. I have never been one to accept things at face value and had to test this for myself. The tailgate is held open by chains which allows air to escape the bed and also creates down force. My hope is that drag (or resistance) will be greatly reduced. This is what I did, and this is what I have found so far.

I fabricated chains to hold the tailgate at an angle so as to allow air to escape while providing down force - much like the wing on a top fuel dragster. Below are pictures of the first round of tests. There is also a link to a You Tube video that does a nice job of showing the behavior of the air over the gate.

I ran 132.2 miles on 6.864 gallons of fuel. Cruise control was set at 75 MPH and I did the initial fueling and refueling at the same pump. There were strong winds from all directions on 4-4-2010 when I ran from the west side of Indianapolis to down I-74 and then back again. All this with a dirty air filter, a MAF that could stand to be cleaned, and a Throttle Body that could also use a cleaning.

I will experiment with different length chains which will create different angles and down force to find the optimum set-up. So far it seems to be working. I like this set-up as I can install it in about thirty seconds and uninstall it in about ten.

I welcome any ideas, opinions, criticisms, or suggestions anyone may have.
 

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Interesting, your theory does make sense. I know Mythbusters did a test of this and found tailgate up to be best (actually, one of the net things were best, but the question is tailgate up or down). However, they did this (I believe) with Jamie's regular cab toyota pickup, so this fits in with your theory. I use a tonneau myself, I like to keep stuff in the bed so this way it stays dry and snow-free. Maybe we should inform Mythbusters?
 

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I have documentation from a few years ago that shows a bed cover will increase the MPG on a 4.6L Ford. I can also document better MPG with my Ram and about 1500 pounds of rock in the back.

I would be very interested as I think i come from the same school of thought as you. Just because the manufacture produces it, it doesn't mean that it is the most effeciant. I have a bed cover on my 10 and have yet to really try a good road trip (either uphill to where I am going or downhill, so i lose it one way or the other). I have a decent trip planned for Wednesday and will track it (I will be driveing the same route for the next 2.5 years).

I have lost mileage with the tailgate down, but have seen different methods of the same idea. One was a triangle that goes against the closed tailgate 9thus creating a smoother transition for the swirling air to exit).

I have seen the tailgate nets (I have also seen spare tires come flying through them when flooring the pedal up a hill).

I have seen flowthrough tailgates (have one on our 93 Dodge Diesel, mainly so we don't screw up the tailgate when we hooked up to a fifth wheel or gooseneck).

Keep us informed.
 

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Discussion Starter · #4 ·
Interesting, your theory does make sense. I know Mythbusters did a test of this and found tailgate up to be best (actually, one of the net things were best, but the question is tailgate up or down). However, they did this (I believe) with Jamie's regular cab toyota pickup, so this fits in with your theory. I use a tonneau myself, I like to keep stuff in the bed so this way it stays dry and snow-free. Maybe we should inform Mythbusters?
Hey Pirates,

I did not get to see the mythbusters follies but I heard about it. I would like to track down the episode and watch it. I heard that they ran one way with the tailgate up and refueled then the other with it down and then refueled again - the run with it up was all down hill and the run with it down was all uphill. From the few episodes I have forced myself to sit through I found that the show lacked any (as in constituted a complete vacuum) scientific rigor. Even so, if the mythbusters only tested a regular cab truck then the study is pretty limited.

I have documentation from a few years ago that shows a bed cover will increase the MPG on a 4.6L Ford. I can also document better MPG with my Ram and about 1500 pounds of rock in the back.
Nice to know that I am not the only one who has noticed this. Thanks for chiming in.

I would be very interested as I think i come from the same school of thought as you. Just because the manufacture produces it, it doesn't mean that it is the most effeciant.
Absolutely correct. The engineers are not the be all end all of theory. Makes you wonder who really controls research and production.

I have a bed cover on my 10 and have yet to really try a good road trip (either uphill to where I am going or downhill, so i lose it one way or the other). I have a decent trip planned for Wednesday and will track it (I will be driveing the same route for the next 2.5 years).
I would be very interested to hear what your results are.

I have lost mileage with the tailgate down, but have seen different methods of the same idea. One was a triangle that goes against the closed tailgate 9thus creating a smoother transition for the swirling air to exit).
Interesting that you mention that because the chains are just for fine tuning the angle and I originally started this by thinking that the open area (the triangle created by the angled tailgate) should be sealed up to channel the air. Maybe I will do an internet search for this. It would be interesting to see what is out there.

I also thought that by running the tail-end air up off the back of the truck that it might help when pulling a trailer. Unfortunately my trailer has been stolen and I am in the process of convincing the police here to help me get it back.

I have seen the tailgate nets (I have also seen spare tires come flying through them when flooring the pedal up a hill).
Yep, something about tailgate nets never sat too well with me.

I have seen flowthrough tailgates (have one on our 93 Dodge Diesel, mainly so we don't screw up the tailgate when we hooked up to a fifth wheel or gooseneck).
Did you see any change in mileage with the flow-through gate?

Keep us informed.
Most definitely.
 

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Flow through gate on the diesel makes no differenc what so ever. We have towed with this thing and get 23MPG at 70. 5.9L Cummins with a 727 (with a wanna be OD) and 4:10 gears, 70 is about all she will do with 8000 pounds behind her, she will however, do it all day long and get 22 - 23 MPG.

The ram never got very good mileage (heavy right foot), but did get better with a load in the bed. \

I will keep you informed on the upcoming trip (about 180 miles each way).
 

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Whatever the mileage, I don't like the look. It kind of reminds me of the guys that wear there pants pulled part way down, showing their underwear. Hey, I wouldn't have said anything but you said criticism was welcome.
 

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Glad to see you are in the mood for more testing. I was going to say I think a rigid brace of some kind would be better than chains, but it seems you are way ahead of me on that. I'm impressed by your mileage so far, with my heavy foot and heavy tires I can't manage that kind of efficiency, but that's user error! Keep up the good work, big :tu: to you.
I'm sorry to hear about the trailer, with all the hard times you have been facing, that is not what you need.
 

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Every truck I ever sat in, with the tailgate up and the rear slider open I've noticed the same phenomenon. The air flow seems to go over the cab, towards the back of the bed, then get reflected back at the rear at the cab. Best reason I can think of why things that are already in the bed and things that float by in the air get blown forward through the rear slider and into the cab.

I am wondering if this air foil creates a sort of push effect on the rear of the cab, counter-acting some of the negative effects of wind drag at higher speeds. And maybe the smoother airflow with the tailgate down which allows the air to flow directly out of the bed without being redirected is not as beneficial as the tail gate up, directing airflow back towards the truck. Just a silly thought.
 

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Discussion Starter · #10 ·
Whatever the mileage, I don't like the look. It kind of reminds me of the guys that wear there pants pulled part way down, showing their underwear. Hey, I wouldn't have said anything but you said criticism was welcome.
YO YO YO YO! Stop looking at my behind.

Glad to see you are in the mood for more testing. I was going to say I think a rigid brace of some kind would be better than chains, but it seems you are way ahead of me on that. I'm impressed by your mileage so far, with my heavy foot and heavy tires I can't manage that kind of efficiency, but that's user error! Keep up the good work, big :tu: to you.
I'm sorry to hear about the trailer, with all the hard times you have been facing, that is not what you need.
Yeah the testing is what keeps me in the neighborhood of sane - sometimes. I will get my equipment back at some point - just very frustrating at this juncture. Thanks for the kudos.

Every truck I ever sat in, with the tailgate up and the rear slider open I've noticed the same phenomenon. The air flow seems to go over the cab, towards the back of the bed, then get reflected back at the rear at the cab. Best reason I can think of why things that are already in the bed and things that float by in the air get blown forward through the rear slider and into the cab.
I think this is due more to the vacuum that forms directly behind the cab and it is greatly amplified when the side windows are down as well. The air rushing past the side windows creates a draft that sucks air in from the rear slider. I learned real early that the window in back only goes down if the bed is surgery prep clean - otherwise anything in the bed ends up bouncing off the windshield and then into my face.

I am wondering if this air foil creates a sort of push effect on the rear of the cab, counter-acting some of the negative effects of wind drag at higher speeds. And maybe the smoother airflow with the tailgate down which allows the air to flow directly out of the bed without being redirected is not as beneficial as the tail gate up, directing airflow back towards the truck. Just a silly thought.
There is an interesting air cycle that forms directly behind the cab and it is most noticeable when you have light debris (leaves or paper) in the bed. It just churns around back there and I am not sure I will be able to stop that. What I am hoping for is to relieve some of the drag while creating down force.

On that note, some of us have noticed better mileage with weight in the bed. The down force approach is attractive to me in that there is no additional weight at take off but the same benefit is realized at speed. I think I am going to try shortening up the chains a little to find that optimum balance between allowing air to escape and creating down force. I will try an install some streamers and run a few tests this weekend with my girlfriend (see signature) driving so I can watch the air flow behavior without pulling a Shatto.
 

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Flow through gate on the diesel makes no differenc what so ever. We have towed with this thing and get 23MPG at 70. 5.9L Cummins with a 727 (with a wanna be OD) and 4:10 gears, 70 is about all she will do with 8000 pounds behind her, she will however, do it all day long and get 22 - 23 MPG.

The ram never got very good mileage (heavy right foot), but did get better with a load in the bed. \

I will keep you informed on the upcoming trip (about 180 miles each way).
No way. I RV it way too much to believe this. My dad has a 03 Ram 5.9l and he gets 20ish unloaded. Once he hooks up to his trailer he is down to the low teens. If you posted this on RV.net they would flame you. Those guys tow and watch there MPG like a hawk.
 

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sr5mike. It is documented. This rare specimen is a 1993 with a 12 valve Cummins. I can vouce for it as we pulled 4ton of wood pellets back from Dolores, CO years back at 70 and documented 22MPG. A bit more than 8000 pounds with the trailer.

About 6 years ago my mother moved my sister from Phoenix to Denver with this truck and a uhaul trailer (looked alot like the Beverly Hillbillies) and documented 22MPG coming up the rim of the Salt River and over Wolf creek pass running 70. We can make a diesel run like a raped ape ('specially a Cummins) and can get this mileage all day long as long as it is maintaned.

RV's are like pulling a HUGE box. Try pulling a flat trailer with a few cars on it your mileage will get better than the same weight in the shape of a box. It is called aerodynamics.
 

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think about it for a second... mythbusters is broadcast on tv! HA! enough said.

i like your approach highwaylizard! i had a 92 silverado extended cab (like the tundra access cab without doors) that gained 3 mpg (18 to 21) with cruise control set at 65 mph with the tonneau cover in place. it is about time for me to revive an old thread i started last fall. i did some gas milage tests with my 06 dc 2wd before installing a tonneau cover. the weather is about the same as it was then so i guess it is time to continue the tests. to finish the tests i'll have to switch back to ethanol gasoline though. :td: and start some new ones too! :tu:

i'm liking the angled tailgate approach you're on to. very unique and cool idea. you have inspired me to look at the tonneau cover in a different way now. my tonneau locks at 1' intervals as it extends to the tailgate... so there are many combinations of possibilities to play with in this regard. tailgate up with tonneau at different positions. tailgate down with tonneau at positions. tailgate angled with tonneau at different positions.

looking forward to read your results. best of success!
 

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I'm definitely going to get a retrax, because they're sexy as hell and fit my needs, but the question to you gents who do a lot of this testing *i'll do some too! dont worry*, is wether or not locking the retrax in different positions will make an impact on gas milage? I'm thinking it will, if i lock it about 2 feet from the cab, it will trap all of the circulating air and probably push it back towards the tailgate, making an air dam that would push down on the rear diff. we'll see though, thats my theory.
 

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Discussion Starter · #15 ·
think about it for a second... mythbusters is broadcast on tv! HA! enough said.
TV MY FRIEND NEVER LIE TO ME!!!

Seriously. Amen brother, amen. I am so tired of the indoctrination.

i like your approach highwaylizard! i had a 92 silverado extended cab (like the tundra access cab without doors) that gained 3 mpg (18 to 21) with cruise control set at 65 mph with the tonneau cover in place. it is about time for me to revive an old thread i started last fall. i did some gas milage tests with my 06 dc 2wd before installing a tonneau cover. the weather is about the same as it was then so i guess it is time to continue the tests. to finish the tests i'll have to switch back to ethanol gasoline though. :td: and start some new ones too! :tu:

i'm liking the angled tailgate approach you're on to. very unique and cool idea. you have inspired me to look at the tonneau cover in a different way now. my tonneau locks at 1' intervals as it extends to the tailgate... so there are many combinations of possibilities to play with in this regard. tailgate up with tonneau at different positions. tailgate down with tonneau at positions. tailgate angled with tonneau at different positions.

looking forward to read your results. best of success!
Wow - thanks. I am humbled to have inspired anyone to anything other than tears. I do not have a tonneau cover to test with and have thought about it for a long time. Please let me know what you come up with.

I'm definitely going to get a retrax, because they're sexy as hell and fit my needs, but the question to you gents who do a lot of this testing *i'll do some too! dont worry*, is wether or not locking the retrax in different positions will make an impact on gas milage? I'm thinking it will, if i lock it about 2 feet from the cab, it will trap all of the circulating air and probably push it back towards the tailgate, making an air dam that would push down on the rear diff. we'll see though, thats my theory.
Very interesting approach to the whole "dance behind the cab" aerodynamics that are going on back there. Maybe somebody with a good sized tool box can chime in on any affects the tool box has on the air or mileage? Atilla keep us posted on anything you find from your testing.


In general to everyone who has posted here with suggestions, questions, and criticisms a big thank you. Lots of good ideas getting exchanged here. It does not matter if they turn out to be dead on the money or just as far-fetched as an honest politician - what matters is that the ideas are being discussed and explored. Very cool.

I shortened up the chains by three links and noticed a few things:

  • The truck seems to ride a little bit smoother and sticks to the turns a little better (Very well could be my imagination)
  • The truck seems a little more "slippier" through the air (Very well could be my imagination)
  • Mileage appear to be up at this point but I will not know until I refuel in the next couple of days.
I got to thinking about the whole closing in the triangle area that is formed with the tailgate at an angle in order to channel the air. I think this could be good in that the angle could be lowered because the focused air would create more down pressure than without the side pieces. I also thought that with the triangle areas open the air rushing past the sides of the bed could draw out the air in the bed through the coanda effect. I think I am trying to strike the balance between optimum down force and minimal drag.
 

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Discussion Starter · #16 ·
Shortened the chains up by three links.

  • ran 126.6 miles
  • burned 6.346 gallons of fuel
  • about two-thirds highway and one-third in town driving
  • average MPG 19.94
Wind conditions here have been windy with 10-15 sustained wind speed and gusts to 30 miles per hour. As a matter of fact the semi-drivers were having a hell of a time keeping it in their own lane. 19.94 MPG is a personal best for me. Taking into consideration the wind conditions I really think 20 MPG could have been realistically attained.

I need to calculate the angle of the tailgate and see where it is at. I am convinced there is an optimum angle at which there is enough down force and enough air is allowed to escape the bed.

Has anybody else tested this yet? I would like to know if anyone can duplicate my results.

On the subject of down force I am thinking this is relevant when you consider that front wheel drive vehicles get pretty good mileage and traction in part due to the engine weight being over the drive wheels.
 

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Shortened the chains up by three links.

  • ran 126.6 miles
  • burned 6.346 gallons of fuel
  • about two-thirds highway and one-third in town driving
  • average MPG 19.94

    I need to calculate the angle of the tailgate and see where it is at. I am convinced there is an optimum angle at which there is enough down force and enough air is allowed to escape the bed.

    Has anybody else tested this yet? I would like to know if anyone can duplicate my results.


  • good results! looks like you are the cutting edge on this subject. i'd be interested in learning what angle your tailgate is at when you achieve tour best mpg's. when i start playing around with this i'll start with the same angel and see if we can get some duplicate results.

    if i remember correctly, the best mpg i got last year was 18.57 mpg. this was stock set up at 32K miles, running cruise control at 65 mph. so our milage isn't too far off from each other to begin with.
 

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Discussion Starter · #18 ·
good results! looks like you are the cutting edge on this subject. i'd be interested in learning what angle your tailgate is at when you achieve tour best mpg's. when i start playing around with this i'll start with the same angel and see if we can get some duplicate results.

if i remember correctly, the best mpg i got last year was 18.57 mpg. this was stock set up at 32K miles, running cruise control at 65 mph. so our milage isn't too far off from each other to begin with.
Hey WhiteLiteNin,

Do you have a Regular Cab, an Access Cab, or a Double Cab? I think the length of the roof in relation to the bed will determine the angle of the tailgate. At any rate, here is a picture of the current set-up. No access to any tools to check the angle but I will try and determine that over the weekend.

Looks like a little less than forty-five degrees. Chain is 13 links plus the S-hook and the quick-link connector I used to take up some slack.
 

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that looks about like a 40-45 degree angle you're working with there. i get what you are saying about the length of the roof in relation to the bed as far as what tailgate angle may be best... and then there's the tonneau cover. toooooooo many variables, or so it would seem. let the fun begin!
 

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Discussion Starter · #20 ·
that looks about like a 40-45 degree angle you're working with there. i get what you are saying about the length of the roof in relation to the bed as far as what tailgate angle may be best... and then there's the tonneau cover. toooooooo many variables, or so it would seem. let the fun begin!
Endless entertaining combination possibilities so I am hoping a bunch of people jump in here, do the testing, and post their results. Could end up being the most effective MPG modification to date in terms of affordability. The chains can be had for very cheap at just about any hardware store or box store.

Yeah, I thought between 40 and 45 degrees but I am thinking about moving it up a little more. Really pleased with the numbers from yesterday and hope to improve a little more on them.
 
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