I predict 15 mpg give or take a couple mpg. Driving speed of 60 - 65 mph sounds about right. My 54 1/2 ton with those rear end gears and original hydramatic trans runs well on the freeway at 70 mph. I would feel like I am pushing it too hard to drive above 70 mph. Remember your 216 engine wasn't designed to be pushed hard on a highway. A 1946 truck wasn't really intended to be driven on the highway at 60-80 mph. The front end should be in good "tight" condition, no loose and sloppy conditions. Front roller bearings, rebuilt brakes, radial tires and three point seat belts would be good ideas. Be sure to inspect the original 16" rims for rusted rivets on the inside, etc. Also important to leave lots of space from the cars in front of you for those unexpected fast stops.
