Interesting, are you tracking regen frequency as well? Is you hand calc MPG over the course of many trips or per tank? Some are reporting that Archoil additive increases mileage. Not turning this into another additive thread but you may want to try a bottle. Also check/clean the MAP sensor.

 

The fuel mileage on my 23 was bad until I got 15k on the truck and is still getting better by about 2 MPG going over same route/routes. Hopefully, yours will get better with more miles and the truck drive train gets broken in.

 

As JimmyD said, your truck is brand new. Let it break in and you’ll get better mpg.
I’d say another 10k or so and you’ll slowly start noticing better mpg

 

Are you a location that has a winter diesel blend?
#1 does not have as many btu per gallon as #2.

 

I'm getting better mileage as I have just over 15k now. This tank is non towing, some city driving, but mostly freeway at 70-75mph. I was almost home when I took the photo, going about 10mph on gravel road. Recent trip with 5th wheel ~16k lbs, 13'5" tall got us 10.5 over 1500 miles going, less coming home with headwinds.

 

I use my 3500 srw hd's to tow the same routes with same trailers everyday. I also keep hand calculated fuel usage of each one. I just traded a 2020 with 72,000 miles that averaged 8.8 , and a 2022 with with 82,000 that averaged 8.1. They also got the exact same milage form the very being.

My new 2025 with 9,000 gets 6.5 with the same everything. i would also say it averages about 1.5 less when not towing as well. Runs great, just uses more fuel.

Is this what others are seeing as well.

I went from a 2017 2500 GMC Duramax which was getting 23 / 24 highway, 14/16 combined, and 12/13 towing 10K pounds, to a 2025 2500 GMC Duramax getting 20 highway, and 9 / 10 combined mph. I haven't towed yet however I'm not expecting anything good at all. I actually think the 10 speed hurts gas mileage not helps mileage. The ten speed trans spins the motor up faster which spins the turbo up faster which calls for more fuel to be injected into the cylinders. My theory anyway. Very disappointed with the 2925 duramax gas mileage.

 

I went from a 2017 2500 GMC Duramax which was getting 23 / 24 highway, 14/16 combined, and 12/13 towing 10K pounds, to a 2025 2500 GMC Duramax getting 20 highway, and 9 / 10 combined mph. I haven't towed yet however I'm not expecting anything good at all. I actually think the 10 speed hurts gas mileage not helps mileage. The ten speed trans spins the motor up faster which spins the turbo up faster which calls for more fuel to be injected into the cylinders. My theory anyway. Very disappointed with the 2925 duramax gas mileage.