Sudden increase and then drop in MPG

Discussion in 'Gen 3 Prius Care, Maintenance & Troubleshooting' started by priusphan, Jul 14, 2025.

  1. priusphan

    priusphan New Member

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    I was torn between posting this in the fuel economy area or here so please let me know if I should move it.

    I have a 2013 prius with 133000 miles.
    I normally get about 47 MPG (Calculating at the pump, not with the onboard computer). A couple of tanks of gas ago I suddenly got about 51 MPG. Then the next tank it suddenly dropped to 37 MPG and has remained low for the past 3 tanks.
    This seems to be mostly for city driving. It still gets 47 MPG on long trips.

    Any help on what might have caused this spike and then drop would be greatly appreciated.
     
    #1 priusphan, Jul 14, 2025
  2. bisco

    bisco cookie crumbler

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    all recommended maintenance done? have you cleaned the egr circuit? tyre pressure at or above spec?

    might be worth checking all the wheels for brake drag, and getting a free 12 volt load test at many auto parts stores
     
    #2 bisco, Jul 14, 2025
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  3. Grit

    Grit Senior Member

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    Maybe more AC use now were in middle of
    Summer. Or late night driving with stock headlamps on. Each lamp
    Is about 55 Watts
     
    #3 Grit, Jul 14, 2025
  4. ASRDogman

    ASRDogman Senior Member

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    Is it the same pump everytime? Or a different one?
    Pumps cut off at different time. They "should" all be the same, but they are not.

    It happens to me sometime. I try to use the same pump at the different stations I go to.
    But it's not always possible. This past few weeks the Prius said I was getting 48mpg.
    When I filled, it calculated to 51mpg.
    The next week it said 47 something, calculated, 43 something.... both times I used a different
    pump then I usually used. It happens from time to time.

    When each time you fill it it lowers, you have an issue. You should look for consistency.

     
    #4 ASRDogman, Jul 15, 2025 at 7:46 AM
    Last edited: Jul 15, 2025 at 4:20 PM
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  5. Mendel Leisk

    Mendel Leisk MMX GEN III

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    Bad gas?
     
    #5 Mendel Leisk, Jul 15, 2025 at 9:27 AM
  6. priusphan

    priusphan New Member

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    Thank you! Tire pressure is good. I haven't cleaned the EGR circuit or checked for brake drag so I might try that. It came on suddenly so brake drag seems like a possibility. I'll also check the 12 volt load. It's a fairly new 12 volt battery (about 6 months old).

    Thank you. I've gone through several tanks from different gas stations so I'm guessing it's not the gas.

    Thanks. I'm calculating based on actual gas used and milage rather than the dashboard info. I've gone to different stations and it's consistently bad milage except when I've gone on longer highway trips at 45 mph or more.
     
    #6 priusphan, Jul 15, 2025 at 3:52 PM
    Last edited by a moderator: Jul 15, 2025 at 7:21 PM
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  7. priusphan

    priusphan New Member

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    Thank you. I've thought about AC use. I definitely have it on more right now but I think last summer was just as hot here in Florida and it definitely didn't drop 10 mpg back then.
    Headlight use is similar to my normal usage.
     
    #7 priusphan, Jul 16, 2025 at 3:14 PM
  8. ASRDogman

    ASRDogman Senior Member

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    All those are good to check and can reduce mpg.
    Since it's not just sometimes you get less mpg.

     
    #8 ASRDogman, Jul 17, 2025 at 7:21 AM
  9. DirkAshburn

    DirkAshburn Member

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    Mine did the same thing. I get 37-38MPG now in my commutes, been steady like that for a couple years now I guess.
     
    #9 DirkAshburn, Jul 17, 2025 at 12:04 PM
  10. priusphan

    priusphan New Member

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    Did you investigate the cause or just decided that is the new normal?
     
    #10 priusphan, Jul 19, 2025 at 5:03 PM
  11. douglasjre

    douglasjre Senior Member

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    Somebody joyriding it while u sleep
     
    #11 douglasjre, Jul 20, 2025 at 1:28 AM
  12. priusphan

    priusphan New Member

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    Seems like the most likely scenario! lol
     
    #12 priusphan, Jul 20, 2025 at 8:23 AM
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