2015 Prius on replacement battery not getting good mpg

Discussion in 'Gen 3 Prius Fuel Economy' started by Roger Elliott, Feb 2, 2026 at 11:05 PM.

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  1. Roger Elliott

    Roger Elliott Junior Member

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    Hi Folks
    I have a 2015 Toyota Prius with a replacement battery (actually 2nd replacement - first replacement was defective and replaced for free). I am running stock dimension tires but they are by a company called Sailun (Trezzo h408 size 195/65). Since the 2nd replacement, in my usual commute of 10miles, stop and go with some hills, I've been lucky to get 40mpg. Usually I would get 42-45 but now I'm getting like 36-39. The car has about 150k on it. Weather is pretty mild usually except in summer when it hits 100 or more. Any ideas?

    Thanks
    Roger Elliott
     
    #1 Roger Elliott, Feb 2, 2026 at 11:05 PM
  2. ASRDogman

    ASRDogman Senior Member

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    2nd replacement? So you got a used battery? Never worth it.
    You need the Sodium Ion NEW battery pack.

    Never heard of that brand of tire. Tires are important to fuel mileage.
    What tire pressures are you running?

    Both will highly effect mpg. Cooler weather effect mpg also.

     
    #2 ASRDogman, Feb 3, 2026 at 7:32 AM
  3. indel

    indel Member

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    Are these battery replacements OEM? Were they replaced by the dealership? Post some results from Dr. Prius app or Hybrid Assistant.
     
    #3 indel, Feb 3, 2026 at 9:32 AM
  4. Mendel Leisk

    Mendel Leisk MMX GEN III

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    Just to clarify: you replaced the hybrid battery, or 12 volt battery?
    Off-topic, but if you haven’t: I would clean the EGR (Exhaust Gas Recirculation) system as soon as possible, and replace engine water pump and thermostat.
     
    #4 Mendel Leisk, Feb 3, 2026 at 10:28 AM
  5. VelvetFoot

    VelvetFoot Active Member

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    So, what do the tires have to do with it?
    You did something, battery, tires, both at the same time? Correlation to cause and effect?
    This is actual mpg, miles/gals, full tank?
     
    #5 VelvetFoot, Feb 3, 2026 at 11:04 AM
  6. BiomedO1

    BiomedO1 Senior Member

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    Are these Toyota guess-o-meter numbers or manually calculated tank to tank averages.

    It's been pretty cold and rainy in Redding and we know how weather affects these cars. Unless you do an apple to apples comparison, under the same circumstances by adjusting only ONE variable - it's difficult to make general comparisons.
     
    #6 BiomedO1, Feb 3, 2026 at 11:23 AM
  7. VelvetFoot

    VelvetFoot Active Member

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    I thought I just said that.
     
    #7 VelvetFoot, Feb 3, 2026 at 11:45 AM
  8. BiomedO1

    BiomedO1 Senior Member

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    I was typing when you hit the return key to post your response..... so I didn't see it.
     
    #8 BiomedO1, Feb 3, 2026 at 1:39 PM
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  9. VelvetFoot

    VelvetFoot Active Member

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    I thought you had me on ignore, or something.:)
     
    #9 VelvetFoot, Feb 3, 2026 at 3:00 PM
  10. Roger Elliott

    Roger Elliott Junior Member

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    HI there
    The numbers I gave were manually calculated at each fill up. Even with the cold and rain and so forth, I usually don't see such a drop in mileage
    Roger

    The Hybrid battery was replaced. The first one was dying slowly - like getting low 30's for mileage but the second died completely, told me not to drive etc. The EGR and water pump and thermostat replacement yo mentioned - those can effect mileage?
    Thanks
    Roger

    No, no. It was a new OEM battery both times. Didn't have to pay for the second one as it was covered by warranty. I thought about a Sodium ion battery but I read a lot of conflicting experiences and thought it would be better to wait till they've been around longer and have their issues sorted out. Yeah I've never heard of the tires either. Read some reviews but there wasn't much experience with them to get a good definitive review. Certainly no reviews about their mileage effects. I usually run 32psi.
    Thanks
    Roger

    They are OEM, but they were replaced by a local Hybrid Shop. Local dealership is way overpriced.
     
    #10 Roger Elliott, Feb 3, 2026 at 8:22 PM
    Last edited by a moderator: Feb 3, 2026 at 11:40 PM
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