I went from a 295/55/20 to the 305/55/20. My mileage decreased before the tire change. The tuning service I use wanted to know what the difference between the Speedometer and gps at those speeds. I understand the dic isn’t accurate it has just been crazy to see it drop over those when it stayed constantly at 18.5 for so long. Sounds like part of it could be due to the lower cetane in diesel now. I also found all four tires at different pressures today ranging from 62 to 80 psi.
You have provided some good data points here. One is the fuel comment. I buy mine at just a few stations here in DFW and a couple on the road to the ranch in NM. YES there is a difference in fuel and it will impact mileage. The other is TP--run all of mine at 70 Cold (adjusted just last week for cooler temps had to add 4 lbs) when running empty and yes they go up a few lbs when hot but no issues even if they are at 80 psi cold and go to 84 or more they are designed to do that.
I have a GPS app that tells me speeds so it is very easy to check been using it my many years because my tires and sometimes wheels are always changed the day after I get the truck because they will not last one trip to the ranch. I ruined one last week in fact 6mm stick in the edge of the tread. Took 3 plugs to get back to Texas.
Try fuel and this is a controversial subject but the additives I use do make a difference. Seen it in RAM, Ford and GM diesels. GM gets the best MPG but the 2020 is the first year in many where GM got their act together so I could buy one that would not get torn up(DEF tank) at the ranch.