I've gotten as high as 20 or 21 all hwy on fairly flat ground. 19 with mostly hwy but a little stop and go and or slight hills. Now though with winter fuel i'm down to 17-18 in these same conditions. The current tank i'm on currently sits at 15.8 with mostly around town driving and maybe 20% hwy.
2018 GMC 2500 SLT L5P Duramax