2008 sudden low mileage - couple of potential factors, but looking for some pointers

I have a 2008 Prius. Purchased in 2015 and has been a great car for the past 7+ years. Generally problem-free. About 180k miles. Last spring while planning for a long summer trip, I purchased the Project Lithium battery upgrade. My thought was that our car was approaching the age where a sudden battery failure would be a huge hassle, especially during a long road trip. So I purchased the batteries, replaced them in early summer, and they were great on the trip. I never noticed a huge increase in mileage, but at worst, mileage did not fall during that time. I have been using the free Dr Prius app from time to time just to watch the status of the batteries, and to my eyes, all still looks as expected, even to this day.

Last fall, we were hit with the theft of our catalytic converter, along with several dozen others in our city. We had insurance cover the replacement and had the work done at what I gauge to be a reputable independent mechanic in our area. Total cost for parts and labor was around $1500.

Our Prius is our secondary car. We have a Leaf that gets used anytime we can, for in-town driving, errands, and any short trips that fall within range. The Prius gets used when the Leaf is already in use, and for longer drives. On a normal week, we might take the Prius out 2-3 times at most. Sometimes for short, in-town use (under 5 miles, perhaps a couple of stops, etc), and then maybe once every other week for a longer drive (maybe 20-40 miles at highway speeds). There are rare weeks where we will only take it out once, so it does sit idle sometimes, but generally it is getting some use on a week to week basis. This has been our pattern since we got the Leaf back in 2018. So I feel like we have a pretty good handle on what mileage to expect.

When we're in town, we always refill the Prius with ethanol-free gas. We have a station that is convenient to refill, and they offer it, so we always take advantage of it. When on road trips, we don't seek out ethanol-free gas, but that's probably 1-2 trips a year that we don't use ethanol-free gas.

This is all background information to the issue. After our catalytic converter was replaced, we noticed several things:

1. Mileage seemed to be significantly lower than previously. Before the cat replacement, we would routinely get 40+ MPG, year-round (some seasonal differences were noticed, but stayed 40+ all year). Within a month or so after the replacement, I started noticing that the mileage was high 20's at best, and didn't seem to improve even after taking some longer drives at reasonable speeds. I hadn't spend much time thinking about this in earnest, but it was put into stark relief to me when I had to make a trip in late December that was about a 250 mile round trip in a 5 hour window to pick up someone from the airport. I fueled up the car before leaving town, drove 125 miles, let the car sit for about an hour, and then drove the 125 miles back home. Our historic averages would have easily netted us 400 miles on a single tank of gas. On this trip, in the last 10 miles or so, I was getting the low gas warnings on the Prius. I kept thinking -- how is this possible?
2. There are a handful of what look like error lights on the dashboard that have been lit for several months now. They were not on immediately after the cat replacement, but I don't recall exactly when they came on/stayed on for good. I assume that means the computer is throwing some sort of error, but haven't gotten it checked out yet.
3. The first trip we took in the Prius (after cat replacement) my wife complained about smelling exhaust. I was never sure if what we were smelling was normal after being without the car for about a month while it was waiting to get fixed. I kind of just assumed that after driving only the Leaf for that time, that she was just being sensitive to the smell and it likely was behaving normally. But I feel like I have been more sensitive to that smell in the months that have followed, but don't really have a reference to know if it is normal or not.

I took the car to a different independent mechanic (import/Toyota specialist) and told him about our mileage questions, and he replaced the spark plugs as he said it looked like they were fouled. I think they probably were, and since that time, mileage seems to be up slightly, into the very low 30's. He didn't seem to be interested to dig into the mileage issue further, so I didn't push it.

I feel like there is still something going on, and I'd like to get it resolved. I don't mind getting my hands dirty (I did the battery installation last summer, FWIW), but I also am happy to find an expert who can better get to the bottom of our issues. I saw the thread for newbies about mileage issues, so I'm posting those questions and my replies below. Any ideas are welcome!

- Have you read this thread Yet?

Yes

- What fuel economy are you getting and how are you determining fuel economy? (trip computer or manual calculations)

roughly now in the very low 30's, according to trip computer

- What fuel economy are you expecting and why?

40-45, as that's what we've been getting for the past 7 years of owning the car

- What are the approximate outside air temps?

I live in the mid-atlantic US. We get cold in the winter (freezing is common), and hot in the summer.

- How long are your trips?

most are under 10 miles, but every week or two we probably get out on a 20-40 miler. 1-2 times a year, hundreds/thousands of miles on a road trip.

- How much of it is city vs. highway? Roughly what's the average speed in overall and and of each segment? Is there a lot of stop and go driving?

Most trips are city (especially short trips) - 25-45mph. Stop and go, but not gridlocked traffic jams. Longer highway driving would be in the 55-60mph range most of the time.

- What region/state are you in? (if you haven't set your location in your profile)

Mid Atlantic US (western Virginia)

- What's the terrain like of your drives? (e.g. flat, gentle hills, steep hills, etc.)

Generally mostly flat. Some occasional gentle hills.

- Is your oil overfilled? (i.e. above the full mark on the dipstick)

No

How old is your 12v battery? What is the voltage reading of your 12v battery after sitting over night?

Battery is about 2 years old. I have replaced it twice in the time we've owned it. We discovered it was sitting too long without being "exercised" after the first time it failed, and since have been pretty good about using the Prius regularly. So if it is low/failing at this point, it is likely not because of low use. I have not yet tested it using that method, but perhaps that is worth a try in the coming days.

Have you had your alignment checked? Any pulling or abnormal tire wear?

Alignment was checked a few years ago. No major issues.

- Are you using the factory tires and wheels? If not, please indicate tire make, model and size (e.g. Goodyear Assurance Fuel Max 185/65R15).

Factory wheels yes. I don't know exactly what tires we've got on it right now. 2 of them are new (replaced last summer).

- What are your tire pressures?

Don't know. But we recently had it to our tire shop as 2 of the tires (the older ones, I think) were not holding air reliably. They resealed them and reinflated and have been trouble-free since then.

- Make, model, year, engine and transmission of previous car? (e.g. 08 Honda Civic Si 2.0L 4 cylinder, manual transmission) What did you actually get on the same trips/commute? (Please give us actual numbers, not EPA ratings.)

I don't have any data on our previous gas car, which we would have gotten rid of in 2018.

- How are you trying to drive (e.g. trying to stay in electric only?) and how hard are you braking?

Try to drive gently generally. It doesn't stay in electric only. No extreme acceleration or braking.

- Are you "warming up" the ICE (internal combustion engine) by letting it idle after powering on?

No significant warmup before driving. Usually less than a minute after powering it on to pulling out of the driveway.

- Are you driving using D or B mode?

D

- HVAC settings? Are you using the heater, AC, auto mode, etc.? If using auto, what temp is it set to?

Yes, using HVAC as needed. For heat (recently) upper 60s.

- If reporting a mileage drop, did anything significant change on your car (e.g. accident, hit a curb or big pothole throwing off alignment, oil change/other maintenance/repairs, changed tires or wheels, etc.) or your commute?

Catalytic converter replacement is the largest thing that someone seems to coincide with noticing the mileage drop.

 

Just tested the battery per the instructions in that other thread -

• Initial measurement with voltmeter, directly at battery, after the car had been off overnight (14+ hours) - 11.85v . Outdoor temps didn't go much below 40 in the past 48 hours.
• In maintenance mode on first run, battery reported 11.8v as well, dropping down to 11.5v if left in that mode for a bit (10 min)
• Turned car on (in "ready" mode) for 5 minutes. Went back in to maintenance mode, shows 12.4v

I assume this means battery is on its last legs? I checked my records, the optima yellow top was installed (by me) in July of 2020. 2.5 years seems short for battery, since the previous one lasted us from March of 2016 until July of 2020 (just over 4 years).

Should I just replace it?