MPG Drop to 35 MPG after New Battery, EGR Overhaul, and Tires (2010 Five/Tech)

Hi everyone,

I’m looking for some insight into a recent MPG drop. I have a 2010 Prius Five (Technology Package) with 180,000 miles. I recently performed a major round of maintenance, but my mileage has dropped from 40 MPG down to 35 MPG.

Maintenance performed at 170k–175k miles:

• Hybrid Battery: Installed brand new "Chinna" cylindrical modules. Dr. Prius test shows 111.82% capacity (Excellent condition).
  
  
• EGR System: Replaced with brand new Genuine Toyota EGR Cooler and EGR Valve.
  
  
• Intake: Cleaned intake manifold and verified all 4 small EGR ports are clear.
  
  
• PCV Valve: Replaced with brand new OEM.
  
  
• MAF Sensor: Cleaned with specialized cleaner.
  
  
• Spark Plugs: Changed at 146k miles (currently have 34k miles on them).
  
  
• Brakes/Alignment: Verified no brake drag and had a 4-wheel alignment 6 months ago.
  

Tire Change Details:

• OLD Tires: Kumho Solus TA31 – 215/55 R17 (I was getting 40 MPG on these).
  
  
• NEW Tires: Bridgestone Turanza QuietTrack – 225/45 R17 (Installed 5 months ago).
  

The Problem:
Even with a perfect battery and a clean EGR/Intake system, I am struggling to stay above 35–36 MPG. My dashboard shows an Average Speed of 18 MPH, indicating heavy city driving.

I know the tire size changed from a very tall 215/55 to a wider/shorter 225/45, and I am wondering how much of this drop is "odometer math error" versus the rolling resistance of the Bridgestones.

Has anyone else seen a 5 MPG drop after cleaning the EGR and switching to 225/45 R17 QuietTracks? Is there anything else I should check (O2 sensors, 12V battery, etc.) that could cause this after such extensive maintenance?

Thanks for the help!

 

Thanks for the reply. I just checked my 12V battery using the hidden diagnostic menu and it is sitting at 10.9V when the car is in accessory mode. I also realized I haven't done a throttle body/idle relearn since cleaning the intake manifold and EGR. I am currently running 35 PSI in the new 225/45 R17 Bridgestones.

Could a battery as low as 10.9V and a lack of idle relearn be the main cause for a 5 MPG drop, or are these tires just too heavy for the Gen 3?

 

Thanks, Mendel. Yes, this is my first time doing a full EGR overhaul (cooler, valve, and manifold cleaning).

Regarding the tires: When I bought the car, it had 215/55 R17 Kumhos on it. I got 40 MPG on those, but I realize now those were almost 7% oversized, so my odometer was under-reporting distance (MPG was likely lower in reality). I switched to 225/45 R17QuietTracks thinking a shorter tire would help, but I've definitely taken a hit.

I also just found two major issues:

1. My 12V battery is reading 10.9Vin accessory mode (Diagnostic Menu).
   
   
2. I cleaned the MAF sensor with a household dry cleaner rather than CRC.
   

I'm getting 40+ MPG on the highway, but only 34.7 MPG in the city (Avg speed 26 MPH). Could the 10.9V battery be keeping me out of S4/EV mode in the city, or are these 225 Bridgestones just too heavy?