Strange pickup aeromod ???

[JasonG]

Well i still need another tank and maybe some (grooaann) ABA testing,but anyway.
I'm a pre OBD diesel so i have to burn 25 gal to get numbers, no scangague here.

I added a full size tool box (pics tomorrow) to my truck when i changed to a 5th wheel and lost the fiberglass cap.
this is the first time in a while i have had a tank with no trailer.
it seems my avg trailerless MPG went fron 17 TO 19 MPG !

Normally:
the air from the top of the cab normally gets caught by the tailgate and rolled towards the cab creating the 'bubble' that guides the air over the tailgate forming a sort of tail.

My theory:
The tool box is rolling the 'bubble' up farther back from the cab. This is both causing the the slope to drop farther back as well as creating a primary shallower angle from the cab to the start of the 'bubble'.

Any thoughts or a nice simulation?

[Zerohour]

Although I've never owned one, Trucks with the Toneau (sp?) covers, or cloth tops over the beds are generally expected to get 1-2mpg more. It eliminates that pocket to trap the air.

With the tool box, if you have a short 6' or a standard 8' and the tool chest is 2' you've conscomed 1/3 to 1/4 of the bed space (respectively). Now the air traveling around the cab will have the least effect on the pocket directly behind the cab. This is due to the general poor design of air flow leaving the edge of the cab (rounded corners). So as the air flow comes back down over the top, and around the side windows it catches up near the rear of the bed of the truck. So basically you filled the location of the worst void. With a smaller pocket near the best part of the air flow you may have eliminated/reduced the drag.

Also if you have loaded that tool box with tools you have carrying additional momentum and maybe be seeing increased coasting distances if you are effectively using hypermiling techniques. An additional .1 of a mile over a few coasts on daily drive can add up to a few miles of coasting under zero engine load. And it could be worth a tenth or more MPG on each fill up.

I know you're talking trailer-less MPG, but now with the 5th wheel, are you towing a trailer? or just have the 5th wheel installed because you have a pickup? (people do this, don't get mad, I'm just asking! :-) ) If you've gone from a trailer hatch hauler to a 5th wheel trailer you may be picking up alot of aero from the trailer shape when you dual haul something back there.

Also to play devil's advocate...

you removed the fiberglass cap. Was it one of those very large, taller than the cab, caps? If so you reduced your over forward cross section. This reduces the drag on the vehicle. Considering the truck is probably 5' at the roof, and the cap could have been ~.5' above you could be looking at 2-4 square feet reduction in the cross section. And that would be worth a few tenth MPGs as well.

Did you drive a different route than normal? I can take several different ways to work, but some have a loss of 3mpg due to town traffic.

Did you use a different gas station? Last I heard the government wanted to lace diesel fuel with 2%-5% bio-diesels....if this is true, could you have found a shop selling straight diesel without tampering?

Have you done any routine maintenance lately? Especially something effecting MPG?

Either way how it worked, congrats on the 2mpg! Thats a big win on a 25 gal tank at your mileage. You're basically saving a two gallons of fuel per fill up!

[aerohead]

Originally Posted by JasonG

Well i still need another tank and maybe some (grooaann) ABA testing,but anyway.
I'm a pre OBD diesel so i have to burn 25 gal to get numbers, no scangague here.

I added a full size tool box (pics tomorrow) to my truck when i changed to a 5th wheel and lost the fiberglass cap.
this is the first time in a while i have had a tank with no trailer.
it seems my avg trailerless MPG went fron 17 TO 19 MPG !

Normally:
the air from the top of the cab normally gets caught by the tailgate and rolled towards the cab creating the 'bubble' that guides the air over the tailgate forming a sort of tail.

My theory:
The tool box is rolling the 'bubble' up farther back from the cab. This is both causing the the slope to drop farther back as well as creating a primary shallower angle from the cab to the start of the 'bubble'.

Any thoughts or a nice simulation?

Jason,if you'll move this thread over to the Aero Forum there's a chance that more truck folks will see it and chime in.

[MechEngVT]

What kind of fiberglass cap did you remove? When I hear "cap" I think of a roof-height box that sits on top of the bed rails and gives the truck a more Suburban-like appearance, or possibly a higher-roof work/camping top like zerohour mentioned.

Even if the cap is roof height and you have no additional cross-sectional area, the cap will make your drag coefficient worse. The large, flat rear vertical wall of the cap plus tailgate gives you a large wake area. With an open bed or a tonneau cover (flat, bed rail height bed cover) there is a tiny wake behind the rear glass and your main wake that is the area of the tailgate. A taller cap gives you larger wake plus more frontal area, destryong your Cd and increasing your A, so your total CdA is much worse.

My personal experience has proven to me that the longer the bed the more important the bed treatment is for aerodynamics. I drive a truck with a 6'-3 bed and have a soft tonneau that yielded 1-2 mpg when I bought it. My brother-in-law has a Tacoma with a 5' or less bed and adding his tonneau didn't make a difference. I once towed a home made 16' flatbed utility trailer with plywood walls and tailgate and I was getting 12mpg at 60mph until I removed the tailgate, at which point I went up to 16mpg at 65.

I think zerohour's hypothesis about the plane of the toolbox affecting how the bed bubble shields the tailgate from contributing to drag area has merit, but short of throwing a bunch of sand into the wind at highway speed I'm not sure the best way to test it. Adding a soft tonneau and watching how the fabric stretches can show you where the low and high pressure areas are, and if you aren't getting a high pressure area (fabric being pushed down) just in front of the tailgate with your toolbox then that must have been what did the trick, and a tonneau might not make any difference.

[JasonG]

The cap wasnt the issue, I saw the MPG increase after I added the tool box. The old #s from the cap I threw out. It added when I had a "bumper pull" trailer. It kicked up a bit in the rear and moved the air over the trailer nicely.

This week I performed very scientific leaf testing. Very environmental as well, the leaves were recycled ! ha ha ha.
They show the tool box has definitely moved the bubble back and created a dead space behind the window. Its like the bed is now 6' long.

Side note, I'd love to move this to the Aero forum, can anybody tell me how?????

[Weather Spotter]

You need to get a mod to move it, I can not.