My speed sensor produces an AC voltage of 5-6 volts - the amplitude is pretty much constant but the frequency changes based upon the speed of the car. The frequency of the AC signal is roughly the same as the speed of the car in kilometer per hour. At 60 MPH the frequency should be somewhere around 100 hz.

This data is for a 91 300D 2.5 turbo but I think it's pretty much the same cruise control system used in your car.