Mileage?

You would have if it were new.
Now you’ll have to go through a major diagnostic effort to try to figure out where the problems lie.
What service has been performed on the car?

 

"Maintenance required" means that it has been more than 5000 miles since someone reset the maintenance reminder, nothing more. At least on my Gen3, it wasn't any smarter than that, and I doubt it regressed from your Gen2.

Here is the Gen2 Low MPG complaints checklist, though it was written for new cars and new-to-Prius drivers. It doesn't cover old car issues, which would start with unwanted brake drag, but is a good starting point. We'll need additional clues to better cover your older car.

Fuel economy complaints/queries? Please copy, paste & answer these questions, esp. if you're new | PriusChat

 

There are so many things that could need work.
Old worn out engine and hybrid battery, poor tires and pressures, dirty throttle body and maf sensor, O2 sensors, bad cat, spark plugs, injectors and etc

 

... and 12V battery condition too.

 

Just had a 3 day trip to Pasadena, CA, Hollywood and L.A., totaled 360 miles and drove back in the rain, with the A/C on to keep the windows clear; filled up 6.144 Gallons with the first click of the pump- that's 58.593 MPG-

 

I have a 2020 Gen4 L Eco Prius. I get right around 55.3 MPG with mixed driving. YMMV

GoodOldBob

 

Sigh ... another website host migration, another batch of broken links in the old material.

Yes, that is the intent. And that is just a first round of queries, no one made a similar formal list for older cars, so we'll have to wing it from there.

As for the 12V battery, yours is low, and that is a known contributor to low MPG, though not likely the only cause. Low voltage does not necessarily mean that the 12V is bad, it may be merely undercharged. In fact, most of them are undercharged. Older generation Prii tend to charge this battery slowly, enough so that units with low usage or just short trips can be perpetually undercharged. Gen2 generally charges slower than Gen3. So if you have a battery charger or tender-maintainer, the first step before any replacement is to get it fully charged. With a charger, keep the current low, no more than 4 amps.

The Gen2 fuel tank (in North American market only) has a problematic and inconsistent fuel bladder, which can really mess up guesstimates of MPG based only on distance traveled and "Guess-O-Meter" (i.e. fuel gauge) readings. The only reliable way to figure MPG is to keep a fuel log book, and compute averages over multiple tanks. Second-best is to watch the car's MPG display, which I believe is slightly "optimistic". Reset the trip meter each time you fill up, to re-start the trip average.

 

How many gallons are you pumping from empty?

 

Was your Prius in READY mode or ON mode when you took the pics? I can't talk about the 12 volt system in the Gen 2 but in the Gen 4 there is a big difference in the 12 volt battery voltage when the Prius is in READY mode compared to what the voltage reading is when in ON mode.
Voltage is only half the story to the 12 volt battery or any battery.

 

Yes, if you can toggle the same way while the Prius is in READY mode ( with engine running ) that voltage reading might be higher. Or if you have a multimeter / voltmeter you could also get a few readings directly from the 12 volt battery terminals, while the Prius engine is running, when it is only in ON mode and when it is OFF.

 

In all Prii, the voltage is much higher in READY mode. The same also applies to all non-hybrids with engine running. That just tells us what the 'charging system' (or DC-DC inverter to 12V bus) is putting out, not what the battery resting voltage is. READY mode voltage doesn't help much in determining 12V battery health.

12V battery voltage is best measured when the car has been OFF a while, at least an hour, overnight is best. Don't make it READY first, because even after turning it off, the voltage will be artificially high for a while.

And how many miles does it normally take to get to that point?

Short trips, cold weather, both contribute to low MPG. Short trips also contribute to undercharged 12V batteries, a chronic problem in older Prius generations.