Here's a data point:
1959 NAPCO suburban, stock 235 engine, 32" tires, 3.38 differential
After 3 hours on the highway 2000-2200 rpm (60-70mph), no braking, mild hills, it averaged 16mpg
Pretty good for an engine last rebuilt in the 1960s with 1/8" ring gaps!
I think stovebolt efficiency is greatly reduced when stop and go is involved... slow driving ~10-20 mph I bet it's below 10mpg.
