Uh, "Warning, Will Robinson," but after talking with an aerodynamicist that teaches auto aerodynamics at the Pasadena Design Center I'd like to point out that these kits are just for looks and will actually hinder the performance of the car slightly.
To wit: first, they add weight to the car which will slow your 0-60 and 1/4 mile times. Second, they add frontal area to the car which actually results in increased net overall drag. (Just a little, but it's really more.) What the manufacturers do to confuse the issue is to claim they lower the "drag coefficient."
Well, that's true, also.
How can this be?
Simple. Drag coefficient is defined by the equation Cd = Total vehicle drag divided by dynamic air pressure (1/2 rho V^2) and the frontal area of the vehicle. Equation-wise, it looks like this:
Cd = Drag/(1/2 rho V^2 S).
(rho is air density in kg/m^3, V is airspeed in meters/sec, S is cross-sectional area in square meters. Drag is force, or Newtons. For you English units folks, rho is slugs per cubic foot, V in ft/sec, and S is square feet, Force is pounds force not weight.)
The area S is the projected area exposed to the airstream, i.e., if you had a light bulb placed an infinite distance away from the front of the car it would project a shadow against a wall behind the car. All of the dark area of the shadow would be the cross-sectional area S used in the equation. Note that European manufacturers do not include the area of the tires where the US manufacturers do.
OK, got it? Now then, note that the front skirt of your add-on aero kit adds frontal area under the front bumper. This increases the car's area S. Because S is in the denominator of the equation for Drag Coefficient Cd, the Cd number will be reduced. Now (according to the aero prof) the front skirt actually adds a bit of drag to the vehicle, but because the area is larger, the net Cd number decreases slightly when you add the skirt, hence the skirt manufacturer isn't technically lying when he claims the drag coefficient is lowered. But in reality the total drag on the car is increased so the car will actually top out at a lower speed.
Also, the 'rear spoiler' thing--Um, it really doesn't do anything there on the rear deck of the S60. Believe me, flow separates off the deck just fine at the existing trunk lip.
Actually, for the optimum condition you want the air flow to separate at the upper edge of the rear window becuase if airflow remains attached down the rear window to the trunk deck then it will form a pair of "wingtip vorticies" and cause even more drag. The tip vortex is caused because of the natural airfoil shape of the car (The car is a low-aspect-ratio airfoil, after all) that generates lift even at zero angle of attack. This is bad, so if you can get the flow to separate (detach from the skin of the car) up on top of the roof line then there's no overall gamma rotation of the air, no vorticies, and hence, no induced drag.
The spoiler on the late 80's Toyota Supra was the best example of good spoiler placement. It had what looked like a roof extension that came horizontally from the roof line over the rear window by about 8 inches or so. It was quite effective in dumping the lift without presenting excessive frontal area to the airstream.
Um. FYI, you understand. Enjoy your new aero kit . . . .
