Frog,
When I was running my '54 Chevy 3/4 ton, with 4.57 rear, and 235 w/ Rochester B, I was averaging about 12 mpg.
If I drove it faster than 50 mph, the mileage plummeted!
My '72 C-10 w/ 250/Rochester B and 3.73 (?)rear, seems to be doing about 16 mpg highway.
Again, if I drive faster then 60 mph, then I can watch the gas gauge visibly fall towards empty
(at 60 mph, the engine's roughly turning 3,000 rpm).
This has been my experience... with the '54, I always used to scratch my head when I'd look at the serial plate on the door pillar which was silver lettering on a blue background: at the top of the plate it said "Chevrolet- for economical transportation"....being a poor kid in high school at the time, 12 mpg didn't sound particularly economical to me !
