I would look at a six cylinder Buick. I don't know for sure that this car would do what you want it to, but I have heard the mpg's are surprisingly high, and I know they are very comfortable highway cars.
Where around Milwaukee can you cruise at high speeds?
Not sure if you are buying new, but here is an interesting 4 cylinder:
http://green.autoblog.com/2009/06/23...0-mpg-highway/
Can it handle high speeds? I don't know.