[elhigh]

I believe a small spoiler at the lip of your trunklid will help you. Like an open truck bed, it will help form a stable rolling bubble of air in the vacuum behind your backlight, and that acts as a sort of Kammback for the rest of the air flowing over your roof. It's far from perfect but better than nothing - which is what you have now.

Obviously some testing would be needed.

I'll second Ecky: I think 40 at 80 is going to be a really, really hard goal to meet. I'm softpedaling pretty hard just to break 50 at 60 (also a HCH1 like Ecky's), and our cars are both smaller and lighter than yours. But if you can do it, you are The Man.

Aero is your friend. Find every advantage you can take, and take them all. I drove a Malibu from Roanoke VA to Boston MA and had a blast. The trunk is like a Las Vegas magic act - all my stuff AND the wife's AND the kids AND the strollers fit comfortably inside, with plenty of room to spare. As a long-distance cruiser goes, it was one of the nicest trips I've ever taken. And as American cars went at the time, it was super thrifty. But still, I never got better than mid 30s. If you can crack that lofty goal I will be very impressed indeed.