I come from engines cultures.
In this area (thematic), it is very important to understand the importance of orifice and the value of orifice coefficients.
For similitude, the central part of a car, it is paragonable to half tube:
below the effusion of inlet and outlet for an tube
look the lowest coefficients of intake similar at horn.
applying the same one in ours cars, we can drop drag coefficient.
For the rearend of car the thing is more complex why the interference with other air flow.
but, this is a whay to consider