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MPG will “work” with an engine up to about 400-cid. And the usual changes to breathe freely. In cars below 5000-lbs, 20-mpg was achievable under 60-mph. (IOW, a V8-413 Dodge, but not a V8-440).
Tow engines were all about TQ at cruise rpm. Gearing was set such that highway speeds were just below peak torque (2800-3200 rpm). Thus if one needed to accelerate, it was easy to do so.
Economy is about highest relative compression ratio exactly matched with camshaft profile and TQ converter stall speed. This is where expertise matters. Can’t really expect to pull it off without a template to copy. Tight quench area also. Low rpm engines under a load are prone to detonation and pre-ignition. The bigger the motor, the easier it is to solve for power.
On highway, keeping a steady pace wasn’t so much about vehicle weight as it was aero drag. And the possibility of more gears than a 3-speed automatic is where drivetrain can be further finessed.
Needed is vehicle weight from a certified scale. The genuine tire rolling height. Guesses will degrade outcome.
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