I've had multiple occasions where it will tell me 999 miles remaining, I'll drive 20-30 miles and when I restart, I'm down to 500. It's for sure not very accurate, especially when it starts getting down the the final ~ 1 gallon or so.
Welcome.. It it likely related to your tuning. Anytime you're tuning these trucks they are sensitive to changes, so I would start by reaching out to your tuner.
Update..
You can see my fuel logs in my signature, but it seems like I've lost a solid 3 MPG by upsizing to these tires... I would post the obvious answer that I didn't buy a pickup for optimal mileage, but still 22-23 is not bad for a full size truck. I will add, we've had horrible weather...
For me the only other maintenance that makes this different from a gas engine is I replace the fuel filters about every 20k or so.. it's cheap insurance. Your standard air filter/cabin filter is by the book, and oil changes are 5K for me. I run the OEM ACDelco oil in mine I get from Rock Auto...
I agree to that.. but it seems like with ANY modern car now they all have something that impacts them. I've learned to just drive an enjoy what I have.. on my EcoDiesel forum every other post people are talking about failures and how it could impact them with that platform.