Back at Thanksgiving I went down to dads in my 06 CC 4x4 3500 CTD. 400 miles at 75 MPH and it got right at 19.8 and still had a 1/4 tank. RPM was high, 2000-2100, I think the rears are 3.70. Good millage for a big heavy truck, tires are off a 2015 275/70 R 18. 134,000 miles, so I can see 22mpg as reasonable