On my 65 corvette pro street we monted an air speed sensor to the bottom of the the car. With the car as low as legal 3 inches clearance at rest, It showed an air speed of only 37 mph under the car at 202 mph measured in the clocks.At this point the nose valence is almost touching the ground. Surprisingly rear downforce was greater with the rear spoiler at only 25 degrees from horizontal. This was measured with shock travel data. Raise the rear spoiler and the car lost mph. This is probably a idiosyncrasie of this particular car but it shows you cant assume anything when it comes to airflow. It does what it does and what may seem an obvious truth my be negated by other unseen factors.