I drive a 2016 Eco and avg 27-28 MPG for mostly highway driving. Mph is 65-70. I tried cheaper Def and dropped to 25-26 MPG and went back to Peak Blue. I have a K&N air filter, but haven't seen a difference in MPG since I installed it about 3-4 months ago. I'm very happy with mine. I change the oil every 6,000 miles and rotate the tires. It's been very reliable.
Sounds about right. Slow down to about 65-68 like me and hit 24ish. With the gde tune I hit 27-29. 70+ kills the mileage crazy fast. Mine is a 4x4.I have a 2015 Ram 1500 ecodiesel with 34,000 miles. 1.55 gear ratio and full size topper. 90 percent of my driving is interstate and about 25 percent of that is pulling a 3,000 lb trailer. I have never once gotten over 21 mpg. Usually average 18 to 19 mpg with out trailer and 12 - 14 with trailer. After having topper installed mileage dropped about 1 mpg to the current 18 - 19 mpg. Never had a code or any other problems. Generally drive 75 - 78 mph.
Anyone have any idea what might be going on? I've bought fuel in at least 10 different states so know fuel isn't the problem. Hope this is not a cam timing gear slip issue.