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Calculated MPG got worse after service?

Discussion in 'Gen 3 Prius Fuel Economy' started by lamontcranston, Dec 15, 2011.

  1. lamontcranston

    lamontcranston Umbra Tenet

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    Hi all,

    Wondering if anyone else is seeing this issue:

    I had my 60k service about 3 months ago. Yes I drive a lot.

    Prior to that I noticed calculated MPG was about 5% higher than actual tank MPG measured by dividing number of gallons used by number of miles driven. In other words, if consumption showed I was getting 65mpg I was actually getting about 62.

    At the 60k service I had the tires replaced as well as all the other normal 60k stuff. Since then I've see that calculated MPG is now 5% lower than actual tank MPG. Again, as an example I calculated 65mpg and consumption showed about 62.

    That's a 10% swing in the consumption measurements.

    I checked to see if the tires were the right size — they are.
    I checked inflation — still proper.
    Since I only use this car for commuting, I can verify that the odometer is reading miles the same as it always has: 58.2 miles door-to-door.

    Was there a software upgrade that might have changed this or is it just some weird thing that just happened? It's certainly no big deal because my actual consumption hasn't changed. In fact since the car hit about 7500 miles, it's been very consistent at about 63mpg in the winter and 68mpg in the summer. I can't complain about that.

    Still... this is the sort of story you post on a forum and hope someone has a pithy comment.
     
    #1 lamontcranston, Dec 15, 2011
  2. stevemcelroy

    stevemcelroy Active Member

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    The new tires alone could account for the drop - from what I remember with new tires you need about 500 miles for them to wear in.
     
    #2 stevemcelroy, Dec 15, 2011
  3. lamontcranston

    lamontcranston Umbra Tenet

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    I'm already at 74,000 and still seeing the issue.
     
    #3 lamontcranston, Dec 15, 2011
  4. Codyroo

    Codyroo Senior Member

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    I don't think Steve read through your post thoroughly (my assumption is that he thought you were complaining about MPG loss, hence his comment about the new tires). It probably had to due with the word "calculated". Most folks on the forums use "calculated" (instead of actual) as a reference to the hand calculation of MPG's (miles driven divided by gallons consumed) and use the term "indicated" for the computer displayed value.

    Wow, interesting development. Did you ask the service dept if they have updated any software on the car?
     
    #4 Codyroo, Dec 15, 2011
  5. lamontcranston

    lamontcranston Umbra Tenet

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    Thanks, I'll use the correct terms from now on.

    My actual MPG is consistent at about 63MPG for this time of the year.
    My indicated MPG, which used to show about 65-66 for this time of year, now shows 61-62.

    I hope that clarifies things and again, sorry for the confusion.

    I have the 75k service coming in about a week and I'll ask the service manager if he knows, but I tend to think you guys here are more knowledgeable than 90% of the service people in the industry :)
     
    #5 lamontcranston, Dec 15, 2011
  6. kswebb1

    kswebb1 Junior Member

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    Height of tire because of the increased tread I would guess.Even when you put the same tire on its larger then the old one due to the tread difference.Also if you change manufacturers of the tire this also can change diameter slightly.

    I put tires on my other car and there was roughly .3 inch difference between one brand to another in the same size.Then add on 9/32 of tread and the difference could be 3/4-1 inch of difference in diameter.

    I would suspect its the difference in size even if its the same tire brand etc.
     
    #6 kswebb1, Dec 15, 2011
  7. GrumpyCabbie

    GrumpyCabbie Senior Member

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    1, Check tyre pressures
    2, Are you 100% sure they used the very expensive 0w20 oil and not the much cheaper 5w30 oil? (then gen3 needs the 0w20 for good economy, whereas the gen2 used the 5w30. Many dealers still mix this up)
    3, Has the temperature dropped lately to account for the mpg drop?
     
    #7 GrumpyCabbie, Dec 15, 2011
  8. xs650

    xs650 Senior Member

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    Holly moly people, has everyone on PC lost the ability to comprehend English?

    The OPS said his fuel consumption didn't change and his odometer vs real distance reading didn't change. All that changed was his indicated MFD mileage.
     
    #8 xs650, Dec 15, 2011
  9. GrumpyCabbie

    GrumpyCabbie Senior Member

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    I haven't got time to read his full post - I'm busy. :)
     
    #9 GrumpyCabbie, Dec 15, 2011
  10. xs650

    xs650 Senior Member

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    :rockon:
     
    #10 xs650, Dec 15, 2011
  11. Mendel Leisk

    Mendel Leisk EGR Fanatic

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    I'm tripping over the OP's language: he's qualifies mileage as being "actual", "calculated" (I always figure those two were the same), and then introduces "consumption". It would be nice if he could clarify. Bottom line, what does in-dash instrumentation say, and what do you calculate?


    Anyway, IF his mileage is suffering post-service, I'd vote it's due to the new tires being different. Again, he's not clear if there was a change in tire make/model.

    Also, "proper" tire inflation is often exceeded around here.
     
    #11 Mendel Leisk, Dec 15, 2011
  12. Codyroo

    Codyroo Senior Member

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    Re: Displayed MPG got worse after service?

    Honestly,

    I think we have had so many posts about "poor winter mileage" or "poor mileage after dealer service" that we automatically assume that the problem is in the amount of gasoline calculated or consumed. We need to actually read the post and try to understand what is being asked.

    The HSI displayed value is much lower than before but the actual/hand calculated MPG's are the same, as is the odometer reading. Would having tires with a different diameter (hence revolutions per mile) explain this? I could see it impacting the speedometer. But would it impact the odometer? Would it impact the HSI displayed reading?

    I understand why people are missing the original question.

    The original poster's (OP) complaint is rather unique. I'd love to find out what is going on.
     
    #12 Codyroo, Dec 15, 2011
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