You guys probably won't believe me, but I get 22-26mpg on the highway at about 80mph with 373's. Around town I usually get at least 16mpg but it can go as low as 10 if i get on it.
Try using a vacuum guage, learn to accelerate with it still pulling a vacuum and go for max vacuum at cruise. I'm amazed at how much gas can be saved just by driving habits. Most of my friends with stock 6 cyl. cars and trucks get worse mileage than me just becase they're too heavy on the throttle.