For my '22 4WD Hemi Rebel bought used about a year ago, it took some cycles for for the EVIC to settle down. It actually reads close to calculated mileage.
Owners manual says 89 octane delivers the best balance of power and economy. I got 17.5 on winter blend 87, did and do now get about 18.5 with summer blend 87. Did run several tanks of summer 89 octane, honestly do not feel more power (really can not use more, this truck can scorch the tires on 87), did get a modest improvement in economy but not enough to justify the extra cost per gallon (by extension, cost per mile). Now back to running 87 which is fine according to the manual.
I might repeat the experiment next winter to see if winter blend 89 is more economical than winter 87. Cost per mile with summer blend is at best a wash.
If I were to do some heavy towing, I would run 89 just to get better ignition advance (in theory, lower operating temperature). I'm OK with these mileage findings, the Rebel replaced my '15 5.7 AWD Charger Pursuit. I got 18.5 mpg calculated (and EVIC agreed) driving it on the same roads I run the Rebel on. This is also the same fuel economy as my '04 Chevy 6.0 RWD with 3.73 rear.
My 6.4 M6 Challenger? Ahem, not as good and I run 93 octane in it. I sometimes see double digits on a long cruise but not when tearing up the back roads.