http://www.roadandtrack.com/go/news/new-technology/keep-that-tailgate-up-mostly

sample quote

Indeed, there was one case,
a 2001 Ford F-150 standard cab,
where lowering the tailgate actually improved CD, albeit insignificantly, from 0.5230 to 0.5215.
(And, yes, pickup trucks are bricks aerodynamically.)

In their report (SAE Paper No. 2004-01-1146),
the researchers discussed underlying reasons for the phenomena.

In general, there's a wake immediately behind any pickup cab, and this suction is greater without a tailgate in place. True, presence of a tailgate adds a small drag increment; but the cab wake predominates.

more info:

http://www.nrc-cnrc.gc.ca/eng/achievements/highlights/2007/tailgate_down_myth.html


http://papers.sae.org/2004-01-1146/

Had a 94 F150 supercab 4x4 about 12yrs ago and drove 45-50 miles to work one way.

Added a tonneau cover, flowmaster catback and K&N drop-in and gained 2mpg highway.

when people ride in the back of your truck bed they find out sitting behind the cab equals dirt in your eyes---riding near the tailgate means cleaner air---so there is a big swirl of air disturbance right behind the cab--maybe an air dam on top of the cab with no tailgate would let air slip by easier....bob

I have a Leer tonneau that I have stored on jackstands. The dang thing weighs at least 300 lbs. There is no way that this thing could make the truck aerodynamic enough to offset its weight.

The high efficiency Ram uses a tonneau and running boards. They did not add them for appearence.

UPDATE:

I added a soft/folding tonneau cover last fall, mostly just to keep the bed clean and the ability to put stuff in back and keep it out of sight/locked tailgate. I haven't watched my fuel mileage that close lately as it's been SO COLD out here that I'm spending a lot of time letting my Ram idle outside to warm up, so my mileage hasn't been great day-to-day.

That said, on Monday and Tuesday I had to drive (for work) over 140 miles each day, all interstate, and decided to check my mileage. So, I filled up at home, drove this trek two days in a row, and filled up at home again. I clocked a 21.9mpg trip with my 2011 Ram 1500 Quad Cab 5.7L Hemi 4x4. I was impressed! I never got that kind of mileage before adding the cover. Now, I don't want to say that the cover did this...but, I was NOT driving these two days in any way to increase gas mileage. 65mph zone, I was treking in at 75-80 most of the time trying to get there quicker...

I'm very happy with the bed cover, and it's certainly not HURTING my fuel mileage!

Thanks for all the replies! I really think that it helps, I don't really know how much, but every little bit counts at these gas prices and the amount of miles that I drive.

JS

Years ago a truck magazine did this comparison on a Chevy, Ford and Dodge. Only the Dodge saw improvement with a bed cover. I want to say it was in the '02-'08 body style if that helps.

The three way comparison of Snugtop tonneau's done in conjunction with SEMA can be read about here:

https://web.archive.org/web/20061003133109/http://www.sema.org/main/semaorghome.aspx?ID=55785

One key point to note is that
the three pickup trucks were only allowed to circle the racetrack at 45 mph
because they did not have fitted internal rollcages,
like the racetrack's liability insurance required.

That is a shame because a test at 75 mph would have more clearly shown the difference in aerodynamics

More from SEMA on tonneau aero on Dodge Rams here from some ASME prize winning engineering students at Western New England College:

https://web.archive.org/web/20050313154211/http://www.sema.org/Main/SemaOrgHome.aspx?ID=52546

A Ford Engineer on how bed covers affect F150 aero:

https://web.archive.org/web/200306031606...s_save_fuel.htm

sample quote

"Tonneau covers on pickup boxes reduce aerodynamic drag"
said Ford's Jack Williams.
"we've seen reductions of about 8 to 10 percent on the F150, which means the average fuel economy improvements for the EPA city/highway cycle(test) is about 2 percent.

"The average steady-state (cruise control) fuel economy improvement at highway speeds is close to 5 percent.."

end quote

On an early walk this morning i saw one of these "Slimline" bed covers on a Chevy pickup and wondered whether the "stepdown" at the halfway point had any aerodynamic effect, considering the US Patent from GM posted above.

http://www.slimlinetruckcovers.net/

The Slimline designer does have aero in mind because there is a mild rear lip spoiler that is difficult to make out in the photo.

On a smaller scale the Rampage topper
made the truck more stable at speed

without it the truck would move around
when big rigs, would blast pass it
on the interstate



at the track it also help the aerodynamics