"Ram air " is a myth, as shown in your attachment. I believe that formula only applies to an isentropic diffuser, and automotive ambient air intakes are far from that, but the intake geometry on a jet engine comes pretty close.

The main advantage of drawing ambient air is lower temperature. Engines gain about one percent power for every 10 deg. F decrease in inlet temperature, so assuming a difference of 30 to 80 degrees between ambient and engine compartment depending on speed and other conditions, the improvement is real.

Duke