Wagon has a 390FE and around town I get maybe 10mpg. I went to the Run to the Sun car show in Myrtle Beach which is ~160 mile trip. Of course I'm expecting not good mileage. Filled up when I got there and was surprised I'd gotten 16.6 Mpg. I was on state highways, not interstate for the trip, so I wasn't sustaining 70 Mph, more in the range of 55 mph.
The key is the rear end gears. It's why I get about the same highway MPG with my '79 Chero--it has 2.47 rear gears.