Because tire diameter and 3rd and higher gear ratios
are involved too,
it is better to discuss this in terms of
Miles per Hour per thousand rpm of engine rotation.

For most heavy V8 vehicles maximum fuel economy on LEVEL highways
comes between 40 and 50 MPH per 1000 rpm.
If your tach is showing 2000 rpm at 80 mph you are in the ballpark.

Manifold vacuum should be
under 12 inches Hg but not less than 4
( you do not want to be in fuel enrichment mode of the Fuel Injection or carburetor)

Camshaft specs matter too. Less lift and less duration help.

One MPG was gained at a steady 70 mph on mostly level blacktop pavement
On a 5400 lb 2wd 360 Ram truck with 39.4 square feet of frontal area and a Aero Cd of 0.42
When the diff ratio was reduced from 3.55 to 3.21.

The $139 computer software modelling program from the Performance Trends company called Fuel Economy Calculator correctly predicted this real world result.

If the diff ratio was reduced from 3.21 to 2.96
Or if the engine displacement was reduced from 360 to 318
the program Fuel Economy Calculator predicts a further improvement of 0.7 MPG at 70 mph.

Another interesting prediction of FEC that I confirmed in the real world was that
if this same pickup is climbing a 4% grade hill,
it does not matter whether you are in 0.69 ratio overdrive gear or 1:1 ratio third gear,
in either gear MPG drops down to 13 because of the added power needed to climb the hill.