[aerohead]

Currently, Bugatti and Koenigsegg are both using air-brakes.
We're talking 258-mph, and 310-mph cars though.
The retarding force, like any other aerodynamic force, is going to vary as the square of the velocity.
I know from observing passenger jet air-braking that, at rather high velocities, the effectiveness of spoilers and thrust-reversers kind of peters out, and they've got to introduce hydraulic braking.
I'm unsure what the breakover velocity would be on an automobile.
At Bonneville, I believe that they want drag chutes to precede friction brakes at around 175-mph, in order not to inadvertently burn the friction material off the pads and shoes.
The owner / driver of the Royal Purple streamliner told me that he shredded both parachutes on a run once, and it took eleven-miles to get the car to coast down to a safe braking speed. Floating Mountain must have looked pretty 'BIG'.