The best I ever got with my 47 coupe 216 and 4:11 gears was around 16 mpg. If this is a major issue you can get "highway gears" from Patricks in Casa Grande, AZ and taller tires, modern carb, electronic ignition etc etc.
Best of luck


It's easier to get forgiveness than permission!
1946 1/2-Ton Chevy
1953 Chevy 3/4-ton Factory Stakebed