Just a stab in the dark .

Take both battery cables from the battery . Hold then together / touching for up to 15 minutes . I read this will force the computer to do a re-learn . Re-install the battery cables .

Go fill the gas tank , to the top and record the mileage . Drive your daily commute until the car is close to empty . Fill it up again . Write down the mileage . Compute the KPL and MPG . Report back .

God bless
Wyr