I too have an ought six Unlimited. With the automatic and a hardtop, which provides a nice quiet ride and I will never remove. Have you ever attempted to reconfigure the aerodynamics? I am making some "pants" for the front fenders to prevent air from getting under them and causing both "lift" and "drag" due to the open front distance between the fender and the bumper. This HAS to affect the handling as well, especially on the interstate. Mine tends to "wander" around 80. My cruise is usually set to 74, but once in a while an "18" gets going along pretty well and I have to get by, because staying beside is not healthy.
I get the standard 15 – 16 in town and 19 – 20 on the road with the mighty AMC 6 with a 55 year track record. No mods of any type, but so far, but I think these pants will be good for 3 – 4 mpg. Cheers, D$D