Speed is a big factor in these old pickups. They were not aerodynamic. Expect fuel mileage to drop with speed. Running two carbs is and a mild cam is not in the interest of fuel economy and you weigh as much as a big Mercedes. Of course, your engine isn't broken in yet. It would surprise me if you ever get up much past 15 mpg if you run 60 or 70 down the highway.
