4 MPG is a little heavy if you were bobtailing. If that was loaded it's par for the course.
I drove a Cornbinder 18-wheel truck-and-trailer loaded with hay back in the day. It had a 350 HP, 1090cu. in. Hall-Scott 6 with a 5X3 trans. It got about 3.5-4.0 for a 150 mile trip, pulling a 3,000 ft. pass. It had dual ignition, OHC, twin updraft carbs the size of coffee cans, two-piece exhaust manifold and very large fuel tanks.
The other truck, a Freightliner with a 220 Cummins and a 10-speed Fuller got about 5.5mpg. It was way slower. Being senior driver I got the I-H and got home a half hour earlier.
