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...