We were lucky enough to scoot in the '17 Limited from OK to FL for the annual vacation with 4 adults, interior cargo deck completely full, equipment rack (platform) hanging off the hitch w/2 tubs of goodies and a beach umbrella. 2,500 miles round trip.
About fell out of my seat when the first tank came back at 19.6MPG while averaging ~75+MPH straight highway. Thought "no way" until the second came back at 20.5. Tank test.
Stayed right at those numbers with the exception of the Ozark Mountains where we averaged around 18.5 on one tank. We even had one stretch of 55MPH that netted just over 22.
Thrilled. And to my surprise the trip computer was never off (higher) by more than a little over 1 MPG. (Our 4th Gen's computer was solid at 2MPG more favorable than actual for 7 years straight) For the 5th Gen, several tank tests were within .5 MPG of the computer and one tank was .1 which is just incredible.
Yes, there could be vast variations due to using different pumps but the numbers were very consistent. My theory? Believe the hitch platform was breaking up the vacuum created by the flat rear of the 4R and thus helped it glide. Because this thing was loaded down and should have been a little more thirsty. Eh, it's what I'm going with.:shrug: