appalling gas Mileage

My Prius has an issue that has been getting much worse. The engine will not turn off unless the battery shows full. I get 25 MPG on the highway and about 30-35 city on a good day. I came here to see if anyone had a similar issue, not completely. However, I did find out about an app called DR. Prius. I have recently started to monitor the traction battery with it and have noticed what seems to be a high base voltage between 220V - 230V. When I accelerate it drops to around 180V, but it has dropped as low as 160V. When I brake it spikes the voltage to nearly 300V. It has been getting worse with time 'about a year now' but has been manageable, so far.

I took it in for an oil change and its 100,000 mile service, in 2019. While explaining this issue with a tech he said that low MPG is common in winter months, which is true, But I still wanted to know if there was a problem since my mpg was lower than prior winters. I asked him to hook up the car and run a full system diagnostic scan of my car, mainly the hybrid system. He agreed. He also told me that there was a software update/patch. I agreed to it and am now regretting it. Since I got my Prius back I have noticed other system issues aside from the drastic drop in mpg. The other big issue is the tank capacity gauge seems to be malfunctioning. It wont go past 413 gallons till empty no matter if I put 7 or 9 gallons in the tank.

I am curious if anyone here has or heard of a similar issue. I am thinking of taking it back to the dealer and having them run the test again, if it shows no issues then have them roll it back to factory software. It is almost as if the update damaged the battery, BMS, ECC or the Hybrid System Controller. Or I hope Toyota isn't trying to pull the same thing Apple did a few years ago with there older phones.

 

When I first got her, her mpg was the normal. After the first 2 years is when I started to notice the drop. It was then just a couple of mpg's so I wasn't that concerned. It was when I took it in for service at 70,712 I really started to notice things.for instance EV mode was now limited to 10 MPH instead of 25. The following winter when it got really cold the engine would start rough as if running dry. The summer of 2017 is when she started to burn oil at 112,602 miles.
The update at 70,712 was "E0E-REFLASH MOTOR GENERATOR & POWER MANAGEMENT ECU-PRIUS"
The latest update was at 134,805. "J0V RECALL, UPDATED POWER MANAGEMENT ECU."
This is the one I suspect is at least partially responsibly for my problem.
So to answer your question "royrose". My problem is like the first hill on a roller coaster, slow at first then a steep drop that makes you wish you were wearing brown pants. I do mostly highway driving with gradually dips and climbs, I use cruise most of the time also. When I start her up cold, she's angry for about 10 seconds the smooths out. No codes yet. I do plan on taking her in soon for them to run tests specifically for the hybrid system. Honestly I would wait on the update but that said if a safety update does come out, I would get it updated.
I will definitely look into the EGR system. I will keep you guys updated and thanks for the info.