It is supposed to reduce turbulence and therefore drag at the rear of
> vehicles and improve the laminar flow thus reducing fuel consumption.
>

Maybe, but probably not noticeably. Digging way back into my memory to
remember a lecture on fluid dynamics from my distant past, I recall that
something similar was used on early aircraft wings to generate vortices that
push the turbulance transition point back a little farther (modern laminar
flow wings don't need these because they have no significant turbulance).
Now on a wing where every square inch of laminar flow means a little more
lift, a little less drag, a few more knots of airspeed off your stall speed,
that's important. But the back of a car? I just don't see it making that
much of a difference. Like you say, a little bit on the highway on a long
trip, maybe. You'd probably save more gas by getting a Barbie head wedged
under the gas pedal, keeping you from pressing it down all the way. And on a
Vanagon, with that big blocky back-side, I don't think a few vortices off
the roof edge are gonna make a fart in a thunderstorm's difference to that
roiling, churning mass of turbulent air we all drag behind us.

By the way, I plan to sell gas-saver Barbie heads for 19.95 on eBay. Look
for 'em...
--
John Bange
'90 Vanagon - "Geldsauger"