Pushing a 8x11' frontal area down the highway at 60mph, you might get 7mpg, maybe 8mpg. Don't sweat it though, even the new ones only get 7 or 8 mpg.
It's a squared relationship between power required to push it, and the speed you are pushing it. Double the speed takes 4 times the power needed to meet the drag force. Basic drag relationship. So if you're willing to drive a little slower you can enjoy the view out your window a lot more and also burn less fuel.