Anybody lost fuel mileage after demandated

[n2racing]

I have 2014 peterbuilt 579 ISX15 2350, 600hp 2050tq. has MM demandate . I immediately lost 1.5 mpg after demandate was done. Anybody else run into this ? I'm 60 yrs old my driving habits are the same before and after demandate. I've ran it a couple months now thinking maybe it was just me. My fuel avarage before 6.8 to 8.2, now it 4.8 to 6.6 (flatbed) bobtailing went from 10.2 to 8.8 mpgs
(1)My max boost is 38-39 lbs
(2)my highest temp egt 9.5 (I70 west of Denver)
(3)the truck has vibration from 1300-1500 rpms when building boost and allways has.

Demandate truck temps runs tons cooler and has better power. Is it possible the better power is costing me fuel mileage ?