Honestly I'm not buyin any of this without more spec's

never saw anything posted about the tranny, rear gears or tire height

I did have a 72 GTO that did achieve about 22mpg hiway, it had tall tires, from memory about 27" and a 4sp with a 3:55-1 rear, the quadrajet also helped, and I had done some WOT bursts in 4th blowing off 3 firebirds, 2 late 70's turbos and 1 early 80's turbo going uphill on 95S from Vermont, I let them try to catch me on the downhill, never downshifted, I had filled up in Ct driven to Vermont and back, refilled and calc'd mileage