If the same volume of air passes through a smaller cross-section or channel, the speed of that air will increase while passing through. It may be counter intuitive but the pressure at that same point will decrease (the Bernoulli Effect). Think of it as a venturi.
In the case of lowering an air dam and the ground, I don't think the volume
will be the same as before lowering, since the gap is more of an orifice than a venturi. If the front of the car channels air under the car (unlike an air dam), however, the airspeed probably does increase and Darin has a good point Doesn't he always?
.