[aerohead]

Originally Posted by Big time

As to measure the whole angle of the car roof, including both the back-window and the trunk

Would the angle between the windshield top or roof upper point to the roof and the upper rear point of the trunk be important?

Something like this


Has this angle been studied by aerodynamicists?

It's the most important real estate on the whole car and has been studied vigorously since the 1850s.
This area,more than any other,can determine flow separation,hence pressure drag (the largest portion of aerodynamic drag).
Here's some R&D