When the national speed limiit was 55mph, Fenner Tubbs Chry-Ply in Lubbock, TX took each one of their demo fleet to do some highway mpg checks. The '74 New Yorker 440 TQ did 20.66mpg and the '74 Newport 400 2bbl did 20.33mpg. They filled them up at the Shell station across from the dealership and then drove them south, out of town, toward the edge of the Caprock, turned around and headed back to the Shell station for a re-fill. Using the factory cruise controls on the highway at 55mph posted speeds. That is what I was referring to, not that you should obstruct traffic.
Take care,
CBODY67