I got around 14mpg in my '70 Duster driving from Charlotte, NC to Augusta, GA cruising at 75-80mph. Stock 5.9L Magnum short block, 9:1 compression, 222° @ .050" hyd roller cam, heavily ported Edelbrock heads, shorty headers, 750cfm Street Demon carb. Around 450 HP based on 1/4-mile trap speeds. I think the biggest draw is the cheap loose 2600-rpm stall converter in front of the 904 trans that seems to slip quite a bit as the car currently has 2.94 gears and 27" rear tires. With an overdrive manual trans that could keep it spinning in the lower-2000s RPM range and no slippage from a torque converter I think it would break into the low 20s for mpg.