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I mostly want to emphasize that the air close to the skin of a car is not the only air being affected. I think people loose sight of this, (or never thought of it really) that as we drive we are constantly moving huge volumes of air, if we look at just 5 feet out from a car and back 300 feet, we're talking about 40,000 cubic feet and 3,200 lbs. Even that number is mind boggling. The ideal car shape will gently push air out of the way and let it go back to where it was with the least amount of energy put into it. Anything that sets the air swirling about or moving along behind the car will have required energy to make it do so, and this energy is what raises the Cd and thus drag of a shape that departs from ideal.
It is the Shape of the vehicle that affects this, air in the "boundary layer" is merely the tip of the iceberg. If you look at the mass of air in the 4 inch area around a car it is minuscule compared with the total air mass being displaced, it is therefore nonsensical to believe that changing the behavior of this tiny fraction of air will somehow result in any thing other than a very slight change in Cd. The boundary layer has no magic powers anymore than a fraction of a percent of hydrogen can change how the engine uses fuel.
6 lbs against 3,200 is just too much.
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