You see it on a lot of trucks. It used to be that a box van was just
that, a box. A decade or so ago, they started rounding the front edges
of the box. Can you say "fairing". Some started rounding the back
corners also. I've seen SUV's with what looks like reverse scoops that
could have to catch the air passing over the top of the vehicle and blow
it down the back. Counterintuitive until you realize that attached flow
offers less drag.

You have been tricked! The attached downward flow has nothing to do with
drag; but is to keep exhaust and also road dirt away from the rear of the
vehicle. Similar ducts became a standard feature of tull sized station
wagons during the '60s and '70s.

The thick tailgate with the rounded edge keeps the air in attached flow
longer. Even a small amount, just a few inches, offers major
improvements over the previous generations 90 degree chop offs.

I have yet to examine the rounded tail gate, but have my doubts. Suffice it
to say that some salesmen are even more creative than the brochure
writers--so you might compare the company litterature to the salesman's
assertion.

Peter