Kawasaki Ninja 500 and 250 have both have a production run of like 15 years and there is good aftermarket support as well. Parts availability is good, they are cheap, and they get good FE. Motorcycle Consumer News got 64 mpg with the 500. They have changed little over the years so a good old one is virtually the same as a newer one. For some reason I like Kawasaki, they usually get good FE for the power and the have more "personality" than refined Hondas that don't get as good FE.
Honda also made a 250 nighthawk for a while, I think it gets 70-80 mpg...it must be a very similar engine to the rebel, and might have better aero. Thats what I am after.
My friend has a 450 nighthawk 1982 and it got 67 MPG when i took it for a cruise last summer. It is fairly fast, faster than most cars under 60mph, and has a 6th "overdrive" gear. The revs at hwy speeds were actually much lower than my old yamaha xs1100.