F = 1/2 * rho * S * Cx * v^2

Where:

* F is the dragging force, in Newtons (N);

* S is the frontal surface of the object (the car in this case), in square meters (m?);

* Cx is the aerodynamic finesse, which is only dependent on the shape of the object;

* v is the relative speed of the object (the car) compared to the fluid (the air), in meters per second (m/s) - in fact it should be separated in vc (object speed) and va (air speed) and written (vc - va)?;

* rho is the density of the fluid, the air, in kilograms per cubic meters (kg/m^3), it's roughly equivalent to 1.55 kg/m^3.

What this tells is that the Cx is only part of the story when it comes to air resistance! For example, take my car, the VX220 Turbo, and a Rolls Phantom. Surely, you can guess that this pachyderm:

has more air resistance than this bathroom appliance:

Despite of the appearances, though, the Rolls has a Cx of 0.385, and the VX220 has 0.41! So the Cx doesn't tell everything, you must also take into account the frontal surface... Which, while it is relatively small on the VX (1.6 m?), approaches the one of a garage door in the case of the Rolls (2.8 m?)! Which gives us an SCx of 0.66 m? for the VX and 1.08 m? for the Rolls...

So, what really matters is the quantity S*Cx, which is the drag coefficient, and often referred to as the SCx...

Also, you can see that the air resistance increases with the square of speed, so you have 4 times more air resistance when you drive 100 than when you drive 50...