"Denny" wrote in message
ups.com...
On Jul 24, 12:20 am, "Richard Isakson" wrote:
"Matt Whiting" wrote ...





cavelamb himself wrote:

It is more fuel efficient to drive a pick-up truck with its tailgate
down, rather than up.

Busted

Driving with the tailgate down actually increased drag on the
pick-up
and caused it to consume fuel faster than the identical truck driven
with the tailgate up. It was later revealed that the closed tailgate
creates a locked vortex flow that created a smoother flow of air
over
the truck. With the tailgate down, the trapped vortex was dissipated
and
the drag increased.

http://en.wikipedia.org/wiki/MythBus...%29#Blown_Away

I remember seeing a wind tunnel test on this many years ago (I think
in
the late 70s when I was an aerospace engineering student) long before
Mythbusters existed. However, there is no way you can convince some
people that this is a myth. They will argue to the death that they
gain
mileage with the tailgate down, however, the wind tunnel tests were
very
conclusive. A rotating vortex is established in the truck once you
get
above a fairly slow speed (I think 30 MPH or so) and this acts almost
like a large balloon in the bed of the truck and directs the airflow
over the tailgate. You can see this pretty easily from the smoke in
the
wind tunnel and the drag change was noticeable also.

Not only do you not get better mileage with the tailgate down ... you
actually get worse mileage! Then again, most people can't even check
their MPG correctly...

Matt

YMMV!

Rich- Hide quoted text -

- Show quoted text -

Paper material in the bed of my truck at 55mph rotates once or trwice
in that standing vortex then vanishes over the tailgate - never to be
seen again...
Any empty, plastic gas can left against the tailgate slides up against
the back of the cab - again that standing vortex...
Now, that the gate up has less drag overal still amazes me, in spite
of knowing what is theoretically happening...

denny

I, for one, am not convinced that a p/u has less drag with the gate
closed--although I concede that many trucks may gain a trivial benefit at
some particular speed.

Their are just too many variables; including the shape of the cab, shape of
the nose, slope of the windshield, and length of the bed; to draw any
meaningful concludions from just one or two tests. In addition, the vortex
grows in size with increasing speed--so that the forward moving portion will
be nearer the back of the cab at lower speeds and may be aft of the tailgate
at very high speeds. However, a radically sloped nose and windshield may
cause the vortex to be much longer front to back at all speeds; the the
lengths of the cab and bed will have an effect.

If I had to wager on the outcome of a *real* test of this issue, I would
guess that there would be a slight drag reduction (on average) with the
tailgate down or removed--especailly if there is any cross wind--and that a
bed cover (a/k/a tonneau) would do at least as well under *all* conditions.

In any case, if you choose to run with the gate open or off, be sure to
secure your load! About 20 years ago, I personally watched a driver lose a
P&W Wasp Jr when he forgot it was there and mashed the accelerator. That
was a major "ah, darn"!

Just my $0.02
Peter