[New Owner Questions] 2004 Prius - Fuel Economy, Battery Swap, Gas Gauge & AC Noise

Hello everyone,

I'm a brand new Prius owner and could really use some guidance from the experienced community here. I just bought a 2004 Prius last month with about 122,000 miles on it. Coming from only driving conventional gasoline cars, I'm completely new to hybrids and Japanese cars, so I have quite a few rookie questions. Thanks in advance for your patience and help!

Here's my situation and my main questions:

1. Fuel Economy - What's Normal?
   
   • Daily Commute: My daily commute is very short—only about 4 km (2.5 miles) one way—with very stop-and-go, congested traffic (long waits at intersections). Using the Car Scanner app, my average trip consumption shows between 5.0 - 8.0 L/100km. For a Gen 2 Prius with a new-to-me hybrid battery, is this a normal range for such terrible driving conditions?
     
     
   • Highway Trip: I recently took a highway trip. After filling the tank and resetting the trip meter, I drove for about 175.9 miles (~283 km) and then refueled. It took 16.5 liters to fill the tank again. This calculates to roughly 5.8 L/100km (or about 40.5 MPG). However, the Car Scanner app showed an average of 4.5 L/100km (~52 MPG) for the same trip, which is a significant difference.
     So my questions are:
     
     1. Is my calculated 5.8 L/100km a normal highway fuel consumption for a 2004 Prius?
        
        
     2. Why is there such a big gap between my manual calculation and the Car Scanner reading? Which one should I trust?
        
        
     3. I'd love to hear from other owners: What are your real-world average fuel economy numbers for both highway driving and congested city commuting? This would give me a much better benchmark.
        
2. Hybrid Battery Swap & ECU Reset
   Last month, I had the hybrid battery pack replaced with a used unit from a 2013 Prius. I've read online that after a battery replacement, it's necessary to perform a "BMS initialization" or "replace hybrid battery" procedure on the car's computer. However, when I asked the mechanic at the shop, he told me it was not needed and that it wouldn't affect fuel economy.
   My question for experienced owners: Is the reset truly unnecessary? Could skipping it actually have a negative impact on my fuel economy or battery performance?
   
   
3. Fuel Tank Capacity & Gauge Accuracy
   
   • What is the actual fuel tank capacity for a 2004 Prius?
     
     
   • How accurate is the fuel gauge? When it shows only one bar remaining, approximately how much fuel is usually left in the tank?
     
     
   • When do you typically refuel? Do you wait for the low fuel warning light, or fill up when there are 1-2 bars left?
     
     
   • My specific concern: I recently filled the tank and drove until only one bar was left. The trip meter showed about 230 miles (~370 km), and it took 27 liters to fill the tank again. This calculates to roughly 7.3 L/100km, which seems high. And because there has only one bar left，Does this mean my Prius can only go about 400 km on a full tank? Or is the gauge pessimistic, and I should keep driving until the warning light comes on?
     
4. Very Loud AC Noise
   The air conditioning system makes a very loud buzzing/humming noise, but it still cools effectively. What is the most common cause for this? Does it require immediate repair or replacement, or is it a tolerable quirk for an older car?
   

Any insights, shared experiences, or advice on any of these points would be incredibly helpful for a confused new owner like me. Thank you for taking the time to read this!