What is the rest of the truck like? Gear ratio, tire diameter, transmission type, torque converter, carburetor type, camshaft grind, all these are part of fuel mileage.
My '37 drops 3 or four miles per gallon if I drive over 65 miles per hour due to wind drag. 250 inline six, 3.08 gear, 28" tires, T350 with tight converter,
21 mpg@ 65 mph, even high at 55 mph.
Install a Quadrajet would be the first move along with HEI and a Air/Fuel ratio gauge.
