[aerohead]

No spoiler is capable of the drag reduction you're entertaining. Only box-cavities and boat-tails can do it.
Remember, your design specification is 60mpg @ 80-mph. And Cd 0.12, half that of Prius, Ioniq.
If it was that easy, they'd already be doing it, and you'd have your 60 / 80 ratio.
The car must be reduced to surface friction drag and wheel drag.
Surface friction we can't do anything about, but wheel drag can be cut by 70% according to Goro Tamai of MIT.