Sorry to break it to you, but you'll be pretty hard pressed to pull 40+ mpg w/ any car in the market right now w/ the way you drive.

I wouldn't try roasting to death all that much. Yeah, AC does have a bad effect on mileage, but if you could only pull off 45+ mpg with the AC off, would you even consider the car anyway?

The FE drop I've noticed w/ AC is about 15% - that is, on trips of the same route in similar conditions, I get about 15% less mpg.

One thing I did forget to mention - my tires are at 38psi. You might want to try that, as there is a hell of an effect at higher speed doing that. Again, in my experience, 80mph should get you over 35mpg.

Based on what you want, have you looked into a Toyota Camry? My old '93 Camry got like 22mpg at 80mph, but it did have a huge gas tank (I want to say like 17gal?).