GM knew aero - look at the early 90s Vectra, 90s Omega, and especially the Calibre for a start.
They even advertised the Calibre as the most aerodynamic car in the world when it was new.
Now unfortunately its in banger-land so it lives with tunnel sized 'zorsts and useless plastic bodykits, just before it goes to the scrapper.
And the tragedy is that they never put the reasonable (for the time) Diesel engines into it. I suspect a Diesel Calibre would have been a 100 mph/40mpg mobile.
Its not just GM though. Do you see VW rushing to make something of the 1L car ?