I have had many of the Cummins diesel 4x4 trucks I will post the years and mileage it would get if driven on the highway.
1989 W-250 Auto 3.07 gear. 22mpg
1990 W-250 Auto 20 plus mpg
1991 W-250 Auto 20 plus mpg
1992 W-250 Auto 20 plus mpg
2002 2500 4x4 Auto 24mpg 50 mph. dropped about 2mpg for every 5 mph. One at 75 mph it pretty much leveled out at 17mpg drove it 150K.
1995 3500 Dually with a dump 5 speed 17 mpg at speeds about 60 mph. I still have this truck.