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Yep, too many variables. But, the '82 F150 I sold had a brand new 351W built by me with a Comp Cams low RPM torque cam, ported heads, polished valves, factory 2bbl intake, 2150 carb, and true-dual exhausts with no cat and glass packs. And, the running gear was a C6 with 3.50's and 30 x 9.5 tires. Running light at a true 60 MPH it got 13 on the highway.
Oddly enough, before I rebuilt it, ported the heads, and put the dual exhaust on it got the same mileage. Yes, the power was much better after the changes, but true MPG didn't change a bit. So, I seriously doubt all those who say that opening up the exhaust saves big on gas mileage. Been there, done that, and saw no change.