My parents had a 1985 5th Avenue that they bought new and before they took delivery of their new car my dad had the dealership install Bosch platinum plugs and with premium fuel dad said that it got 30mpg on the highway. It had the lean burn system and a 904 transmission, no overdrive. I've heard of others getting in the mid 20's with the 5th Avenue and they are as aerodynamic as a shoe box.
