Bad Gas mileage - 12v battery solution

First Post here. I have a 2015 P-IV bought 2018 as CPO in California. I was getting about 45-46, which was fine with me with the driving distance/type involved. Moved to Texas after 6 months and the mileage dropped to 40-41, but different driving environment/climate/distances. Of late since early Aug, it has dropped to 30-32. There are no error codes thrown or any different lights. The only change is I did a one-off refuel from 7-11 (as opposed to my regular place) and then the drop happened started. Battery tests at 11.9 from inside and 12.1 at the terminals. I am thinking bad fuel or battery going bad. Repair on inverter recall done about an year ago, tire pressure is at 35, filters are clean, Oil change done 3000 miles ago (2 months), car is well maintained mechanically. Battery is generally installed new during CPO sales so it's at least 3.7 years old. Suggestions on this MPG drop are welcome. Although will do a refuel with STP additive and clean MAF sensor + throttle body just in case. Oh and overall 80K miles on the odo.

 

7-11 fuel is NOT the best. It's the cheapest from whoever has the cheapest.
12.1 volts at the battery is not good. Is there still a warranty on the battery?
You could but a 4 amp or less charger on it and see if it will hold a higher charge.
According to other PC members a low 12 volt battery will cause all kinds of problems.
You only have 80,000 miles on it? It's a little early for the EGR Circuit cleaning, but close.

Clean the sensors, put in the stp fuel treatement to helpt get rid of the water in the fuel, fill
the tank with GOOD fuel, charge or replace the 12 volt battery, and see if the mpg's improve.
You may have to do a 2nd stp treatement to make sure you get all the water out.
Then run a bottle of Marvel Mystery Oil in the fuel. That will clear out the injectors.

 

I've learned from replacing two previous 12V batteries that the car will tell you, when that battery is dying.

Just listen to what I suspect is a small pump under the hood, which kicks on for less than a minute, when you shut off the car. It normally has a buzzing sound, but when the 12V is getting weak it will sound like an old 56K dial up modem trying to connect to the server.

 

posting an update (alas, not one that is positive). So after getting the 12V battery checked at two places, refueling with the STP additive twice, maintaining 35 psi - there is no huge discernible change in the MPG. Going back to my fuel history, the drop started after a trip I made to Houston in which I drove for about an hour at 30-35 mph in torrential rain. First time for me in that situation. Would such a driving situation cause a lingering issue impacting mileage? Water accumulation or penetration somewhere?

 

Are you basing your MPG numbers on the display........or on your own actual calculations ?

Driving in heavy rain can produce extra drag and hurt your fuel efficiency.
IF.....you have a long run of bad MPG and do not reset the display, that bad result will stay in the long term average forever.

And finally, under what conditions were you testing the battery voltage ? With the onboard system or an external voltmeter ?
Regardless of the "Auto Zone" test results, those numbers still indicate that the battery is suspect.......depending on how your measurement was done. But a weak battery almost never exhibits only poor MPG as a symptom.