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?

 

Thanks, bisco. I don't know exactly how old the 12V battery is, but seeing 10.9V on the MFD (Multi-Function Display) makes me think it has a dead cell. I’m going to take it for a load test today, but I expect it needs replacement.

The driving environment is the same as before—mostly city and suburban stop-and-go. However, I moved from a very tall/narrow tire (215/55 R17) to a wider/shorter luxury tire (225/45 R17 Bridgestone QuietTrack). I suspect I’m feeling the combined hit of the wider tread and the loss of the 'odometer bonus' I was getting from the oversized 215/55s.

Should I also be worried about the MAF sensor? I cleaned it with a dry cleaning solvent instead of proper MAF cleaner. Could that be contributing to the 34.7 MPG I'm seeing now?

 

Thanks, bisco. The highway MPG being 40+ makes me think the EGR work and the new hybrid pack are solid.

I suspect the city MPG is suffering because the 10.9V battery is preventing the car from entering the S4 stage (engine-off at stops). It seems like the ICE is running much more than it should at low speeds, likely trying to maintain that weak 12V bus.

Regarding the tires: the previous 215/55 R17s were so oversized (26.3" diameter) that I think my old 40 MPG was an 'optimistic' reading. Now that I’m on 225/45 R17s (25.0" diameter), the odometer is spinning faster, so the 'drop' on the screen looks even worse than it actually is.

I’m going to replace the 12V with a Walmart EverStart Platinum AGMand get a new Denso MAF to be safe. I'll report back if that restores the city EV behavior.

 

UPDATE: 12V Test Results and OBDII Live Data (MPG Drop 2010 Prius)

Hi everyone, thanks for the previous advice. I have some data updates and would love your thoughts:

1. 12V Battery Status:

• Resting Voltage (Home): I measured 10.9V in accessory mode.
  
  
• AutoZone Load Test: They rated it as "GOOD, Needs Charge."
  
  
• State of Charge: 66% (325 CCA).
  
  
• Ready Mode Voltage: 13.2V to 14.2V.
  
  
• Question: Even if it passed the load test, is 10.9V resting/66% SoC enough to keep the ICE running more frequently in city traffic (Avg Speed 26 MPH)?
  

2. OBDII Live Data (At Warm Idle in Park):

• Coolant Temp: 190°F - 192°F (Seems healthy).
  
  
• O2 Sensor 1 Equivalence Ratio: 14.57 (Seems perfect).
  
  
• MAF Air Flow Rate: This is inconsistent. I saw it jump from 2.29 g/sec in one test to 7.0 g/sec in another, both while at a warm idle in Park.
  
  
• Note: As mentioned before, I cleaned this MAF with a household dry cleaner. Does 7.0 g/sec at idle suggest the sensor is over-reporting and over-fueling?
  

3. Tires & Math:

• Confirmed I am on 225/45 R17 Bridgestone Turanza QuietTracks.
  
  
• My previous tires were 215/55 R17 (oversized).
  
  
• I now realize the 5% diameter drop means my current 34.7 MPG display is under-reporting actual distance, while my old 40 MPG was likely over-reporting.
  

The big question: With a healthy engine (192°F) and healthy hybrid pack (111% capacity), is this 5 MPG "loss" simply the physics of the wide 225 luxury tires combined with a "lazy" MAF sensor? Should I replace the MAF despite no codes?