I have been getting about 17 on the highway and sometimes up to 19 depending on speed according to the indicator in the ceiling liner of my 2007 chassis cab with a service body. This is on the interstae with a not very aerodynamic 102" wide service body. Pulling a trailer it varies but is usually about 11-12 with a heavy load on a 25 ft gooseneck. The engine is very quiet compared to my 2002 5.7. I am happy with the mileage in the application I use the truck.

The 6.7 has an amazing amount of power and I am getting better mileage than on my 5.7 (or is it 5.9?) running in similar conditions. I think if you had a standard bed on the back you could probably get close to 20 on the highway at 65 mph.

The Cummins engine manual says that you don't get the best mileage until after about 8000 mi on the odometer. That seems to be the case and maybe why there are differences of opinion. I am close to 20,000 mi on mine. With this type of truck especially its hard to talk mileage without talking about the truck weight, add ons and duty cycle. Mine was a chassis cab. With a 50+ gal std fuel tank it goes a long ways. Its usually loaded down with near 100 gal in the transfer tank and lots of weight in the service body.

Jim at Dawn