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?

 

Thanks for the detailed feedback, ASRDogman. I want to clarify my 12V data points to be exact:

1. The 12V Battery:

• Resting Voltage: My multimeter shows 12.42V when the car is completely off.
  
  
• Under Load: When I put the car in 'Ignition-ON' mode (engine off), the car's diagnostic menu shows it dropping to 10.9V - 11.1V.
  
  
• AutoZone Result: Their machine (on AGM setting) said 'GOOD, Needs Charge' with a 66% State of Charge.
  
  
• It seems the battery is healthy at rest but has low reserve capacity under load.
  

2. OBDII Live Data (Steady Warm Idle at 190°F):

• MAF Air Flow: Steady at 2.29 g/sec .
  
  
• Fuel Trims: My LTFT is 3.91% and STFT is -3.13%.
  
  
• Hybrid Battery: Testing at 111.8% capacity.
  

3. Tire Correction:
I confirmed the current size is 215/50 R17(Bridgestone Turanza QuietTracks). My previous tires were 215/55 R17.

• The old 215/55s were 6.9% oversized, making my old 40-43 MPG reading very 'optimistic' due to odometer error.
  
  
• The current 215/50s are 3.7% oversized, so the display error is smaller now.
  

I have Reset Trip A and pumped the tires to 42/40 PSI. Based on the healthy 2.29 g/s MAF and 3.9% LTFT, it looks like my engine work was a success. Is the remaining MPG 'drop' just the reality of these heavy Bridgestone tires?

 

I also want to add that a few months ago, right after my alignment, several dash lights (VSC/ABS) popped up. I had to use Techstream to perform a Zero Point Calibration and clear the codes. Given that my battery is currently resting at 10.9V under load, I suspect the battery was so low during the alignment that it triggered those communication errors.

Does this confirm that my 12V is the 'hidden' reason the engine is refusing to stay in EV mode in the city?

 

Thanks again, ASRDogman. I checked the sticker and it says A23, so the battery was manufactured in January 2023. It is about 3.5 years old.

I haven't put it on an external charger yet, but as you suspected, the shop load test yesterday said 'GOOD, Needs Charge' with a 66% State of Charge. My home multimeter shows 12.42V at rest, but it still drops to 11.1V as soon as I put it in IG-ON mode.

Regarding the tires, I confirm they are 215/50 R17Bridgestone QuietTracks. I moved to these from 215/55 R17s. I’ve reset Trip A and bumped the pressure to 42/40 PSI.

My engine data seems very healthy: LTFT is 3.9%and MAF is 2.29 g/sec at a warm idle (190°F).

Do you think a 3.5-year-old battery at 66% charge is weak enough to be my main MPG thief in the city, or should I be looking elsewhere since the engine trims look okay?

 

Thanks, ASRDogman. I’ll follow your advice and back the tires down to 38 PSI to stay on the safe side.

To answer your questions:

• Hybrid Battery: It is not Toyota OEM; I installed a new set of cylindrical modules. I've tested it with the Dr. Prius app and it shows 111.82% capacity, so the high-voltage side seems very strong.
  
  
• EGR System: Yes, I replaced the EGR cooler and valve with brand new Genuine Toyota parts and deep-cleaned the intake manifold (including the four small ports) at 170k miles.
  
  
• Spark Plugs: I changed them at 170k miles with NGK 93501 (Laser Iridium).
  
  
• Coolant: I replaced the engine coolant when I did the EGR work. My OBDII shows the engine stays steady at 190°F - 192°F.
  

I’ve Reset Trip A and I’m going to put the 12V on a smart charger this weekend as you suggested. Given that the engine is healthy and the hybrid battery is strong, I suspect I’m just fighting the weight of these Bridgestone Turanzas and the math of the oversized 215/50s

 

Thanks, ASRDogman. That comparison is very helpful. Since you are getting 42 city/48 highway on 15-inch wheels with efficiency tires, it makes me feel much better about my numbers.

I'm realizing that my 17-inch Technology wheels combined with the 215/50 R17 QuietTracks(which are not LRR) are simply a heavy setup. I'm likely never going to see 45+ MPG with these 'heavy shoes' on the car.