[ECONORAM]

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.

I realize this thread isn't brand new, but I'm disappointed they did their testing at 45 mph. Really? They should consider their customer base. How many do they really expect to drive 45 mph down the highway? I suspect they chose 45mph because much above that the drag really starts climbing...at least on my pickup it does.

Originally Posted by Christ

I agree.

I find it difficult to quantify improvements for a specific truck against other trucks without some context, because each model has varying cab height, bed length, etc.

What specifically works for one truck may be totally off for another. The only concrete point is that they all are affected by the same rules, but the workarounds are often different to a degree.

Good point. If (if) I can find a way to easily remove my topper without destroying it, I'll try to do some A-B-A tests for my RAM. I am curious to see how close it'd be to the 1994 Ford results. Or maybe some A-B-C-B-A tests with C being a bed cover config...

Originally Posted by aerohead

I dug through the rat's nest looking for square bedcover-related material.
I found two articles which may share some insight and one personal experience to share.
(1) When the all-new Chevy/GMC pickups entered the market around 1987,their Cd had been trimmed from 0.535,to 0.45.
In 1992,the Chevy Suburban K 1500 was reported with Cd 0.39,about 11% drag reduction.It's station wagon style roofline closely mimics a traditional square camper shell.
(2) When the VW Vanagon loses it's roof to become the drop-side pickup truck,it's Cd grows from 0.42,to 0.46 ( about 8.5% difference)and with identical power,the top speed falls from 83 to 78 mph.
(3) When John Gilkison owned the T-100 which I now have,on astronomy camping trips,a LEAR square type fiberglass shell was reported to increase fuel economy by up to 1.2-mpg,a 4.8% mpg improvement,and 8% drag reduction.

Interesting data on the GMC Cds. I'm a little surprised at the T-100 results.