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Old 09-04-2012, 08:38 PM   #1 (permalink)
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Help! quick and dirty 'tarp topper' to aero pickup?

I have a pickup that gets a rather horrific 14-15mpg on the highway with the taller-than-the-cab topper it currently has. I need to move it 250 miles sometime soon. I'm trying to figure out if there is some kind of quick and dirty mod I can do to give it better aero... like a quick metal frame clamped to the bed and a tarp giving it a single slope from cab down to an arrow point on the extended tailgate or something. But it's possible flapping in the breeze will be so loose it will create drag unless it's really tight.

I plan on driving slower but dont want to be a road hazard. I don't need to see out the back for years my original topper didn't even HAVE windows, I just need cheap and quick since I don't plan to use it on the highway much at all after this one time. (it will be parked awhile)

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Old 09-04-2012, 08:42 PM   #2 (permalink)
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Triangular plywood topper coming down at a 15* angle on top. Make sure the edges don't stick out past the cab.

A tarp has the possibility of flapping vigorously in the wind, negating gains.

You could also just cover the rear half of the bed, like a half tonneau cover. This would leave the forward half next to the cab open. GM filed a patent on it in 1984- covering only half may give better gains than a full tonneau.
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Old 09-05-2012, 10:59 AM   #3 (permalink)
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single-use to save less than $7 ... worth it?

Is it really worth the effort for one 250 mile trip?

How much time & work are you willing to invest to save less than seven dollars? (I'm assuming saving money is your goal.)

The math:
  • You'll use 17.86 gallons @ 14 mpg (250 miles / 14 miles per gallon)
  • Let's say you improve your fuel economy a reasonable 10% with a quick & dirty aero shell.
  • That means you'll save 1.786 gallons, or $6.79 at $3.80/gallon.
You could probably save more than that just by choosing an alternative route that lets you drive slower without disrupting traffic.

I think Sven's advice on the half-tonneau is a good compromise on return on investment for this case. Could probably slap something together out of used building materials in 15 minutes. Throw on a partial grille block, fold back the passenger mirror, etc., etc., etc.

(PS - I also read recently that the bed needs to be enclosed under that rear half-tonneau for best effect, meaning: there needs to be a vertical piece at the forward end between the bed and the tonneau.)
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Old 09-06-2012, 08:08 PM   #4 (permalink)
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Quote:
Originally Posted by Sven7 View Post
Triangular plywood topper coming down at a 15* angle on top. Make sure the edges don't stick out past the cab.

A tarp has the possibility of flapping vigorously in the wind, negating gains.

You could also just cover the rear half of the bed, like a half tonneau cover. This would leave the forward half next to the cab open. GM filed a patent on it in 1984- covering only half may give better gains than a full tonneau.
What about covering the half rear of the bed, and also having a slope down from the cab on the front half?

Quote:
Originally Posted by MetroMPG View Post
Is it really worth the effort for one 250 mile trip?

How much time & work are you willing to invest to save less than seven dollars? (I'm assuming saving money is your goal.)
Good point. Guess i'm too compulsively thinking of saving, like coupon cutters who buy stuff they dont need. XD That said i'm also wanting to actually start experimenting with ecomodding more seriously, learning is part of it, i'd probably apply it to something else in the future, plus I have scrap wood laying around since it sounds like that works better than the tarps anyway.
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Old 09-06-2012, 08:47 PM   #5 (permalink)
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Quote:
Originally Posted by stillsearching View Post
What about covering the half rear of the bed, and also having a slope down from the cab on the front half?
Half tonneau or full aero cap... I don't really think there's a worthwhile middle ground.

You could just drive 2 seconds behind a semi the whole way and get better FE improvement. My Rabbit usually does 32mpg but when I slipstreamed on a highway trip I ended up with a 36mpg tank... and that includes 50 miles of "spirited" driving!

At 60mph and 3 seconds behind a truck, the surrounding air is going 10mph slower, as if you were driving 50mph. Try it! Also, X2 on folding back the mirrors, using a grille block etc.
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Old 09-07-2012, 05:46 AM   #6 (permalink)
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Quote:
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Half tonneau or full aero cap... I don't really think there's a worthwhile middle ground.
Okay. Does anyone have any rule of thumb of what mileage improvement I might see from each option?

And what additional improvement blocking grille with cardboard and folding the mirrors in?

Or just plain driving as slow as I can without being arrested as a road hazard? :P
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Old 09-07-2012, 06:13 AM   #7 (permalink)
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Quote:
What about covering the half rear of the bed, and also having a slope down from the cab on the front half?
My understanding of the half-tonneau is that the air whips around and pushes on the back of the cab. The one would cancel the other.

Quote:
You could just drive 2 seconds behind a semi the whole way and get better FE improvement.
I like a flat bed with a half-height load. Less buffering than a full-height van.
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Old 09-08-2012, 12:37 AM   #8 (permalink)
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improvement

Quote:
Originally Posted by stillsearching View Post
Okay. Does anyone have any rule of thumb of what mileage improvement I might see from each option?

And what additional improvement blocking grille with cardboard and folding the mirrors in?

Or just plain driving as slow as I can without being arrested as a road hazard? :P
The 1988 SAE Paper from the folks at Texas Tech did a full breakdown for drag with different tonneau cover and cab-wing combos,and with both short and long bed pickups.
GM's half-tonneau U.S.Patent has a drag vs tonneau length breakdown included.
If you can get a hold of these documents you'd have something to chew on.
Perhaps some of the members can help.I'm about 500-miles from home right now and have no materials with me.
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Old 09-08-2012, 03:39 PM   #9 (permalink)
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I'll try to dig out the paperwork aerohead sent me. I don't recall it being more than 5 or 10%. Check back tonight.

Don't expect quick fixes to get you much over 10% either. With something like an old F-150 you'll be lucky to break 20mpg without major mods. For only 250 miles, drive behind a semi. Look at my Rabbit gas log- from normal driving habits to a slipstreamed highway trip I added 12.5% MPG. In a truck that's 2mpg if you're lucky. Like $8 savings.

Let's say your time is worth only $10 an hour... is making something complicated really worth it?
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Old 09-09-2012, 11:20 AM   #10 (permalink)
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Okay.

US Pat 4,573,730. 4 Mar 1986

Chevrolet C-10 tested with different length half-tonneaus. 50% of bed length gives greatest benefit at ~0.045 Cd reduction. For other trucks it may vary- other lengths of 40-60% may suffice. The illustration shows not only the half-tonneau on the rear half of the bed, but a vertical piece at the front of the tonneau that boxes in that rear half.

Example not in the reading:


SAE Paper 881874, 7-10 Nov 1988

"Pickup truck drag reduction-- devices that reduce drag without limiting truck utility"

Figure 1


Full cover is rated at 0.030 Cd reduction.

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Last edited by Sven7; 09-09-2012 at 12:57 PM..
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