Like Brian said, no one number is correct for all cars. Mercedes spent a fair amount on wind tunnel research, so the drag coefficient is lower than on other cars, i.e. they are more efficient than some other cars.

However, the HP required to push anything thru the air is a geometric relationship---doubling the speed requires FOUR times the HP to overcome aerodynamic drag, tripling the speed increases the HP requirement by NINE times.
Also remember that aerodynamic HP is but a part of the complete picture of what it takes to move a vehicle on the road. Tire resistance, mechanical drag, friction in the drive train are some of the other factors.

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1982 300SD " Wotan" ..On the road as of Jan 8, 2007 with Historic Tags