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MPG Dropping

Discussion in 'Gen 3 Prius Fuel Economy' started by Thadeus Buttons, Aug 27, 2014.

  1. Thadeus Buttons

    Thadeus Buttons New Member

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    So I bought a used 2010 Gen 3 with ~26,000 miles on it in July 2013. I started out getting 48-52mpg depending on where I was driving. I live in Raleigh, NC and I did notice a drop in drop in MPG over the winter ranging between 44-48mpg but the mpg never rose again as the months warmed and I averaged 44-46mpg for most of the summer. After changing my tires a few weeks ago I've dropped to about 40mpg regardless of where or how I drive. I bought Pirelli P4 Four Seasons 195 65R15 91T and have them inflated to the levels recommended by Toyota. 35/32 or something like that. Is it the tires or is there some other possible issue?
     
  2. bisco

    bisco cookie crumbler

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    it's probably the tyres. did you get low rolling resistance? if it's not the gyres, it might be a bad 12 volt battery. welcome to priuschat!(y)
     
    #2 bisco, Aug 27, 2014
    Last edited: Sep 3, 2014
  3. xpcman

    xpcman Senior Member

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    You tires are not LRR and they also need a break-in period. I suggest you try 38/36 or even 40/38. Those pressures will probably increase your mpg somewhat.
     
  4. Thadeus Buttons

    Thadeus Buttons New Member

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    So how long should I expect it to take to break in my tires?
     
  5. bisco

    bisco cookie crumbler

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    4-5,000 miles. no guarantee they will improve tho.
     
  6. Thadeus Buttons

    Thadeus Buttons New Member

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  7. bp1000

    bp1000 New Member

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    5000 miles to break in tires?

    Should be done by 500

    I'm sure a 20% drop from changing tires is a little excessive?
     
  8. bisco

    bisco cookie crumbler

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    that's why i would check the 12v.
     
  9. Thadeus Buttons

    Thadeus Buttons New Member

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    So like a bad battery in my battery pack?
     
  10. bisco

    bisco cookie crumbler

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    no, the 12 volt starts the cars computers when you power up. it's in the hatch, right fender.
     
  11. bisco

    bisco cookie crumbler

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    put a digital voltmeter on it in the morning before starting the car. if it's under 12.4 volts, pick up an oem at toyota and replace it. 2010's are notorious for 12v problems, and having work done on your car (new tyres) can exacerbate it because they leave the door open while working, the dome lite stays on and drains the battery.
     
  12. Thadeus Buttons

    Thadeus Buttons New Member

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    Ok I just wanted to make sure that it was the normal starter battery and not something in the pack.
     
  13. bisco

    bisco cookie crumbler

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    yes, when the 12v is low/bad, the hybrid battery keeps charging it, leading to the ice recharging the hybrid battery more which lowers your mpg's.
     
  14. retired4999

    retired4999 Prius driver since 2005

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    I would guess tires, check side walls on tires for max psi settings. My tires are at 44 psi front and 42 psi rear.
    Do not inflate over max psi. Run 2 psi less in rears (Toyota recommends). You will get 3-5 MPG more.
    Good luck!
     
  15. Thadeus Buttons

    Thadeus Buttons New Member

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    I got the battery tested and it's good so I don't think that's the issue. I'll find somewhere I can air up the tires more. The max pressure seems to 44psi.
     
  16. bisco

    bisco cookie crumbler

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    if that doesn't help, let's revisit the battery.:)
     
  17. Thadeus Buttons

    Thadeus Buttons New Member

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    What should I set my tire pressure to if my max PSI is 44?
     
  18. bisco

    bisco cookie crumbler

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    personal preference, mine are 42/40, f/r. sometimes you have to weigh ride quality vs mpg's.
     
  19. Mendel Leisk

    Mendel Leisk Senior Member

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    ^ It's up to you. With the higher profile 15", even at 44 psi the ride's won't be too rough. Experiment. I'd say 40~. Going the extra 4 lb's to 44 isn't going to gain you signif mpg, and will impact the ride.
     
  20. Thadeus Buttons

    Thadeus Buttons New Member

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    I had the dealership look at the car and computer to see if they could see anything obviously wrong but they didn't. Sucks that they didn't find anything but it was free so whatever. I also had them rebalance and set my pressures for F40/R38. Now to see what happens.