Test: F-150 pickup "cap on" vs. "cap off" @ 60 mph
 

Found an interesting efficiency test done by an owner of a 1994 Ford F-150 pickup.

His goal: "to finally (we hope) determine the best configuration for fuel mileage" between...

• bed cap on
• cap off, tailgate down
• cap off, tailgate up

Just an observation about this little experiment:

The canopy on that truck is very boxy, which obviously will increase its drag. Most of the canopies on trucks (atleast, most of the ones I see) are rounded off, and have a cleaner profile than this one.

Also, look at that space between the cab and the canopy... That's a pretty large space, especially towards the top. It's known that the size of the space between the bed and the cab affects the aerodynamics of the truck, and I'm sure the canopy is no exception.

Don't take my comments in a bad way, it's a good experiment! It's especially good for the owner of the truck, and people with the same or similar truck and canopy. I just wanted to point out a couple of things that could change the results that, I'd say, the majority of other people would get if they did this. I plan to do a similar experiment this summer, except I plan to do it with a week, or a month, for each different setting.

None of the regulars here are suprised, are they?

I'm not, :)

Don't you want an abrupt ending to air's flow so that you get cleaner air separation?

I was sure that my cap is better than the bare truck. But now i have to check it.

yes, you ideally want a "clean break" at the rear, but the front edges should (again ideally) be rounded.

In this test I don't really think that having the leading edge being "boxier" would effect the outcome very much. Seems to me that the results are pretty clearly showing the effect of trailing wake.

With the shell on, the trailing wake is as big as it's ever gonna get, therefore the mileage suffers.

With the shell off and tailgate up, the trailing wake is smaller, and (I think) the air can best approximate the "ideal" flow, coming down from the cab at approximately 13 degrees, just clearing the top of the tailgate.
Clearly it's better to not have the air "approximate" it's ideal line, but to give it an actual form (aero cap) to flow over.

With the tailgate down I would think that the air is no longer being "fooled" into following the 13 degree angle, so it's just separating off the back of the cab much like it would separate off the back of the camper-shell.

I believe mythbusters busted the tailgate up down scenario..

however, my other truck did so much better with the tailgate down, I could actually get it upto 90 much faster (3 second difference) with it down vs up..

but it wasn't a traditional cab on frame with bed setup.
http://i4.photobucket.com/albums/y10...T/MVC-274S.jpg

when I took it to the drag strip to dick around with it there,
I deleted the tonneau, tailgate, bumper, inner bed panels, spare tire and spare holder..

shedded quite a bit of weight..
with a whopping 60whp I did good at the track doing bracket.

F bomb warning..
in the in car video, we were all making fun of the fast n the furious.. ;)
Looks like that MF stayed up..
the camera was wedged in the pass seat headrest.
http://www.youtube.com/watch?v=ziNx0x_YXVw

Hi Barwelle - welcome to the site....

IMO, the biggest problem with the cap shown is that it appears to be wider and slightly taller than the profile of the cab. Add to that the sharp corners on those projections, and yes, it's going to be worse than a new cap.

But probably not dramatically worse. The problem, as andrewj pointed out is the size of the trailing wake - that's where most of the drag is. With a cap - rounded corners or not - it's going to be big, nearly 100% of the size of the projected frontal area.

I'd love to know what you're driving and how you plan to test it.

Note that testing by the month or tank will produce much less accurate data than these guys got, because you can't control for weather, temperature, different trip profiles (speed, traffic) etc.

I believe it's dangerous to take as gospel anything busted or confirmed on that show :)

A more scientific analysis suggested that, on average for a range of trucks tested, tailgate up is best. But in reality it depends on the vehicle: SAE report: Pickup Truck tailgate aerodynamics... better up, down, or off?