What mileage does your slant six powered ride get?

A-body, 225, 904, 3.23, 2bbl Carter with 2-1/4 exhaust. Gets 15-18mpg on road trips with two people and luggage, mostly back roads. Would get higher on the freeway if I kept it at 60 or less, but that never seems to happen. Aerodynamics is a b****.