Does a traditional square pickup camper shell topper help or hurt drag & mileage?

[aerohead]

Originally Posted by MetroMPG

Colour me surprised. I heard back from the company whose "documented tests" I referred to in post 14. The results were posted on the vendor's site after all. (In my defense, it would have been useful if the page I originally found that mentioned the tests had a hyperlink to the relevant page!)

Have a read - their experiment is documented here. Their test methodology beats the heck out of Pop Mech's. But it still could have been improved - easily.

They did A-B testing of a cab-high cap vs. open bed on a 2004 Dodge Ram 1500 SLT Quad Cab with a 5.7L Hemi V8. They used a closed course, and cruise control at 45 mph for 50 miles of each condition.

Here's an example of their cab-high cap:



Their claims:

19.38 mpg (US) - cap on
18.06 mpg (US) - cap off

... suggesting a 7.3% increase in MPG with the cap.

(They also document testing two tonneaus on two other trucks.)

The two most obvious pitfalls to their testing were:

1) it wasn't A-B-A, and,

2) they relied on the AAA's official "3-click" method of filling of the tank to determine fuel use (I don't care whose filling methods they were following to the letter, that approach still opens things up to potentially significant errors).

EDIT: the other important omission from the test is that it doesn't say whether the cap is better or worse than a tonneau.

To completely fill my father's Diesel Olds and Caddies,it could take up to 10-minutes and 100-clicks before the fuel would stand steady below the detente in the filler neck.( Really a -itch on a cold windy day in West Texas!).
I give the thumbs down to AAA 3-click.

[MetroMPG]

But clearly,the aeroshell design appears to be the undisputed heavyweight champion for low drag in pickups.
There is nothing published in the public domain to suggest an architecture which exists,that will do more for reducing drag in a pickup.
And there is nothing in Fluid Dynamics which suggests an architecture which could.

Don't get me wrong: the purpose of this thread wasn't to suggest the camper style top could be superior to a properly designed tapered aero cap.

I was just very surprised to read from several sources that it was possible a conventional topper could improve fuel economy compared to an open bed. It seemed counter intuitive to me...

Originally Posted by aerohead

Back when pickup Cds were on the order of 0.50,adding a traditional topper would cut the drag of a pickup to that of a station wagon or Carry-all,Suburban,etc.,at around Cd 0.45.With this you might expect a 5% mpg improvement( HWY).

... but we seem to be collecting a fair amount of anecdotal and imperfect test evidence that it may be true.

[saddlsor]

Originally Posted by Frank Lee

Your window cleaning airfoil might have negated any aero benefit.

huh? I saw no difference with the camper shell or without. by adding the airfoil my mileage went up. that's why I was thinking about a kammback type of foil to see if it would be better. also thinking about something on the sides to spill more air behind the truck. am I mis-interpreting what you're saying?

[Frank Lee]

Typically window-cleaning airfoils increase drag. I don't know why yours would be different.

[aerohead]

Originally Posted by MetroMPG

Don't get me wrong: the purpose of this thread wasn't to suggest the camper style top could be superior to a properly designed tapered aero cap.

I was just very surprised to read from several sources that it was possible a conventional topper could improve fuel economy compared to an open bed. It seemed counter intuitive to me...



... but we seem to be collecting a fair amount of anecdotal and imperfect test evidence that it may be true.

Before 1973,there is very little case-specific data for pickup trucks.The SAE papers just lump everything together.
The early Suburban,which is a direct descendant of the Chevy/GMC C-10 pickup,would mimic a pickup with a straight topper.The Blazer was also a pickup derivative.
I'll look deeper,but my recollection tells me there is little fertile ground in the past for specific data.
PS the El Camino/Ranchero might be compared to their sedan/coupe/wagon counterparts for clues.Perhaps some of the members have data on those.

[Christ]

Originally Posted by bondo

Christ, in cross section these things do happen. Also the proverbial bubble of air is produced in an open truck bed to trick the air in the boundary layer into not entering the bed.

Tonneau covers do improve fuel efficiency. Camper shells do decrease fuel efficiency, as proven in wind tunnels.

The following link has argument sustaining both sides of the camper shell issue. There is a link in the link to a study done in a wind tunnel on how a camper shell is worse aerodynamically but I could not get it to open.

How much does putting a camper shell on a pickup truck impact gas mileage? | Answerbag

Bondo

I should mention here that there were a few assumptions made in that statement, as well:

The cap fits snugly as an extension of the cab.
The truck bed doesn't extend back into the flow aft of the cab roof. (small, short bed pickups)

And, of course, that was not my research showing anything, it was purely intuitive, and explains why people might think that the cap is better than other options.

Take it as you will.

[MetroMPG]

FYI, I never did hear back from Snugtop on my question about the discrepancies between what appeared in the video and the description of the test results on their web site.

[jamesqf]

Just to add some real-world data, I put the shell on my pickup in winter (mainly so I can leave skis &c in it). Here's the tank mpg as I've been tracking it. Don't keep track of dates, but the first tank is probably in November.

# m g m/g
1 16.243 404.8 24.92
2 15.168 383.7 25.30
3 15.440 402.8 26.09 <- Cold & lots of snow starts
4 14.197 351.7 24.77 <- Shell on about 2/3 of tank
5 15.350 405.5 26.42

We finally got good snow towards the end of tank 3 (and also very cold for here temps) so mpg goes down on tank 4 'cause I was driving up the mountain in snow/4WD almost every day. On #5, the weather has warmed up and I'm back to not skiing every day, so tanks #2, #3 and #5 would be much the same driving pattern.

So from this it looks like a cab-high shell might be marginally better than an open bed, at least on my '88 Toyota.

[KamperBob]

Jamesqf, wow. A 22-year old 4WD truck still getting 25+ mpg sounds great.

Rock on
KB

[Christ]

Not the most scientific test, but still excellent inclusion to the thread, James.