I think I've commented here in the past that I recall in the late '60s one of the big car rags tested the downforce on a '67 Camaro Z-28 with no spoiler and with the stock spoiler at 3 or four different speeds. At that time the numbers and the photos at speed, indicated that for the Camaro, the spoiler started providing measurable downforce just above 100 MPH. Camaros were, of course, always light in the tail.
This was LONG before wind tunnels began to influence design of course. Oh well, not long to wait now.