Just returned from a 2,000 mile trip from Monterey CA to Sequim WA. with the 600 Cummins auto . Some Hiway 5, 55 to 75 mph., some 101 at 35 to 55 mph. Did the same last year with the 6.9 Ford with the Banks turbo. Both 4x4 trucks are set up the same except I went with the automatic this time on the Dodge. The 87 Ford has only 120,000 miles since I bought it new. I had the same camper for both trips. Alaskan 2,000 lb. Hyd. up and down.The Ford has the 3.55 gears and will cruse at 70 mph on Hiway 5. I got 13.5 mpg with the 6.9 and 13 mpg with the Cummins. I drove at a higher speed with more comfort and a safer ride with the new Dodge. The hills melted away in OR and I kept the AC on to work the engine as it seemed to not even be working hard. My wife and I love the ride and the height of the Dodge and are very pleased with the power. When the engine is fully broken in I'm sure the 600 will match or better the MPG's of my old 6.9 International. I kept the old Ford for the dirty work and put the new one in the garage.