Best possible mileage?

My 83 slant powered short bed D150, a833 OD transmission with 3:73 gears will get 15 mpg in every day city stop and go driving. On the highway, keep the speed at 65 or below it will get in the low 20’s mpg. Push it it to 75 and above and the mpg will drop to the 17’s. I could probably get better milage numbers, as I don’t drive this truck ez.
The 80’s D150’s are great trucks, but they are also heavy and as aerodynamic as a brick.
About anything on the road today will get substantially better mileage. To get the best mileage keep the tires properly inflated and drive it with a light foot on the throttle.