2021 AT4 Duramax 3.0L low mpg (~17mpg) per computer

[Nickyd1224]

Any update on this beowulf77?  I have a 2020 AT4 3.0 and my mpg has always been way less than what others are getting.  My truck has a little over 8k miles and currently the mpg is around 18.  From the day I got it, I figured that it was a little low, but would eventually get better.  Best mpg I ever averaged was around 20.  I commute to work, very little if any city miles, and my average is still crap.  I purchased the 3.0 for the increased mpg, but now I'm wondering if I should have stuck with one of the gasers.  Hopeful you figured something out on yours.

[Millardoo]

I am having the exact same problem with my 21’ AT4. Just purchased, it was a lease return. Cruise control on a flat highway best I’ve got was 18. It’s leveled with 275s but that should t make that big of a difference?

[Delson0]

Stock 21’ AT4 w/ the 20” wheels in CA (I’m sure I’ll diesel is garbage like our gas is). Snugtop cap with approx 250lb of items in the bed at all times.

Mixed driving 22-23 mpg. Buddy just got the 22’ Chevy. He’s getting 28ish mostly highway.

[Millardoo]

Any ideas on what maybe be causing it? Empty bed highway driving flat roads like 17MPG. Even downhill I’m barley in the 30s. 

Edited December 3, 2022 by Millardoo
Previous statement was made in error.