Best possible mileage?

I have a 84 D-100, slant six 833OD. I got the truck from the family of a friend that passed, and don't know what, if any thing was done inside the engine. It does have the super six two bbl, and a better exhaust. The rear has been changed to a 8 3/4 3.91 sg. Only checked the mileage once, from Florida to Ky, towing my race car. Truck was just the way I got it, except for a safety inspection, and oil change. Never even checked the timing. Got 13 MPG. Same trip with my 88 D-150, 360 4bbl/727 got about 9 mpg