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Fuel econony - cap vs no cap

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Old 05-23-2005, 04:16 PM   #1
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Fuel economy - cap vs no cap

Weight vs aerodynamics? I have a 1993 22RE 4x4 pickup. The cap is fiberglass, circa 1988, and pretty heavy. Maybe it's all in my head, but since I put it back on a month ago, the truck feels heavier... I just started a new job 24 miles away, 97% highway, and I want to maximize fuel economy. Am I better off with less weight without the cap, or more aerodynamic with?
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Old 05-23-2005, 04:42 PM   #2
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i think the only way to know for sure is to do the experiment yourself. try it for a month each way and see what your milage is. generally it used be thought that the cap would help with aerodynamics more than the extra weight. but i saw an article (can't find the link now) where an engineering student put a full-size chevy in a wind tunnel and found out that the way the truck was designed, it was actually better without the cap. he did some other tests and found that the results were highly dependant on the truck. so there's no way of knowing unless you do the test yourself on your truck in your driving situation.
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Old 05-23-2005, 06:47 PM   #3
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Quote:
Originally Posted by mike_d
i think the only way to know for sure is to do the experiment yourself. try it for a month each way and see what your milage is. generally it used be thought that the cap would help with aerodynamics more than the extra weight. but i saw an article (can't find the link now) where an engineering student put a full-size chevy in a wind tunnel and found out that the way the truck was designed, it was actually better without the cap. he did some other tests and found that the results were highly dependant on the truck. so there's no way of knowing unless you do the test yourself on your truck in your driving situation.
I remember I reading something like that too.
They did tests for like a year with the tailgate up and down, and the "bubble" or "cushion" (forgot what they called it) that was created with the tailgate up let the air fly right over it contrary to what many people believed.
It also gave the truck a more stable ride somehow with it up. I don't recall what they said about it, and I never owned a pickup so I don't know how true it is. Just passing on some stuff.
I'll look for it when I get minute.
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Old 05-23-2005, 07:01 PM   #4
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Originally Posted by 4-RUNNIN' FREAK
I remember I reading something like that too.
They did tests for like a year with the tailgate up and down, and the "bubble" or "cushion" (forgot what they called it) that was created with the tailgate up let the air fly right over it contrary to what many people believed.
It also gave the truck a more stable ride somehow with it up. I don't recall what they said about it, and I never owned a pickup so I don't know how true it is. Just passing on some stuff.
I'll look for it when I get minute.
i dont know about the camper topper but there used to be an article about the tailgate on the TTORA-NE website... i just looked and the site where the writeup used to be does not exsist anymore
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Old 05-23-2005, 07:28 PM   #5
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I have run with the tailgate up and the tailgate down. I think I get better gas mileage and performance with the tailgate down. I haven't tested mileage either way, but I am pretty sure the tailgate down gets better.
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Old 05-28-2005, 11:17 AM   #6
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I'm going to run two tanks through it with the cap on, and two with the cap off and see which way gives me better MPG.

I have heard that it is illegal to drive with the tailgate down. Anyone else heard of this?
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Old 05-28-2005, 12:37 PM   #7
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Car and Driver had an article on this several months ago. The best aerodynamics and mileage were acheived with a tonneau cover.Aerodynamics were the worst with the tailgate down. They stated that it is best to leave your tailgate up. By leaving it down there was more lift in the rear that destablized the truck at high speeds. I have a tonneau cover on my truck that cost about $200 new. Well worth the money, it keeps the bed of my truck clean, and it weighs about 10lbs.
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Old 05-28-2005, 06:23 PM   #8
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Quote:
Originally Posted by Cardiac
I'm going to run two tanks through it with the cap on, and two with the cap off and see which way gives me better MPG.

I have heard that it is illegal to drive with the tailgate down. Anyone else heard of this?
i've never heard of anybody getting pulled over for having the tailgate down.
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Old 05-28-2005, 06:31 PM   #9
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i've never heard of anybody getting pulled over for having the tailgate down.
maybe if they have the lic plate on it like a 4runner but a pickup never heard anything
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Old 05-28-2005, 07:23 PM   #10
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i definitely get better mileage without my cap on my bed...by about 2 mpg.
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Old 05-28-2005, 10:28 PM   #11
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Quote:
Originally Posted by Cardiac
Weight vs aerodynamics? I have a 1993 22RE 4x4 pickup. The cap is fiberglass, circa 1988, and pretty heavy. Maybe it's all in my head, but since I put it back on a month ago, the truck feels heavier... I just started a new job 24 miles away, 97% highway, and I want to maximize fuel economy. Am I better off with less weight without the cap, or more aerodynamic with?
GM sent all of us a bulletin at work last summer about people asking if leaving the tailgates down equals better gas mileage. They did their own wind tests and concluded that the gate up was actually less wind resistant than down. It doesnt make any sense to me but the engineers get paid for stuff like this so I wont argue. If I can find the dang notebook I put the bulletin in then I will try and get the source for you.
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Old 05-29-2005, 01:27 PM   #12
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Truck Trend has an article about this in their May/June issue. Conclusions from the National Research Council Canada, Ford Motor Company, and DaimlerChrysler.Driving with the tailgate down decreases highway fuel economy, and removing the tailgate usually makes it worse.In many instances, it also incraeses rear lift at speeds, an undesirable condition in a vehicle with only a third of its weight back there to start with.Putting a tonneau or hard bed cover over your truck is the best thing you can do for highway fuel economy. A shell can also help, since most are relatively light. It also states that the tailgate adds rigidity to the box and was never designed to be left down, as the hinges and cables are not good shock absorbing devices and damage can occur to the tailgate and box.

Last edited by TACOMANATOR; 05-29-2005 at 01:30 PM.
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Old 05-29-2005, 06:47 PM   #13
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Hmm. All very good info. A tonneau cover has been on my wish list for a while now, but lately, so has food. I think I will remove the cap once I can afford a liner and tonneau cover. Right now I have nothing protecting the bed except for the cap. I have a good sized hole starting in the floor of the bed, but I have a patch for it, which I'll put in one of these weekends...
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Old 05-29-2005, 07:05 PM   #14
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i read a summary with pix of a guy who did his masters thesis on this. his results confirm the tailgate-up recommendation, and that a tonneau cover helps as well. he then continued the wind-tunnel experiments to find that a kind of weird, angled cap that started at around the top of the rear windows and ended at approximately the top of the (closed) tailgate helped economy even further. i can scrounge a bit and possibly find the article if someone wants to PM me. i might have bookmarked it.

Last edited by photoleif; 05-29-2005 at 07:06 PM.
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Old 06-06-2005, 09:49 AM   #15
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I would be interested in that link. I may be getting a 1998 Tacome Pre-Runner. I won't be purchasing it, father-in-law wants to give it to me. I need to be able to put my dog in back, but I want some way to keep him in, and to keep my stuff in without it having the possibility of walking away from the truck.

Hey photoleif, sent PM.
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Old 06-06-2005, 01:04 PM   #16
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Quote:
Originally Posted by Ironmike4x4
GM sent all of us a bulletin at work last summer about people asking if leaving the tailgates down equals better gas mileage. They did their own wind tests and concluded that the gate up was actually less wind resistant than down. It doesnt make any sense to me but the engineers get paid for stuff like this so I wont argue. If I can find the dang notebook I put the bulletin in then I will try and get the source for you.
of course it makes no sense, it came straight from a GM engineer. if they were smart they would be w/ ford or toyota (maybe dodge). they are full of it
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Old 06-06-2005, 01:09 PM   #17
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I know on the highway you can FEEL a difference with the gate off....although I never measure MPG with the gate on vs off I'd say it would have to be better with it off...
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Old 06-06-2005, 01:16 PM   #18
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Alright, I'll put the end to this, I have done this test with the tailgate up/down.

I had drove 450KM on $50 of gas with the tailgate up.
I had drove 450KM on $40 of gas with the tailgate down.

These numbers are the average of the 2 times I did each one (4 trips total), all highway km's.

As far as a topper I can't speak for, as well as a tonneau cover.
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Old 06-06-2005, 09:44 PM   #19
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the original link was dead, but the web archive saves the day. http://web.archive.org/web/20030414190338/http://mars.acnet.wnec.edu/~ehaffner/did.htm
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1997, aerodynamics, airodynamics, cap, cover, drive, illegal, massachusetts, mpg, pickup, rack, tacoma, tailgate, thule, tonneau, topper, toyota, xcab

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