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Avg. Fuel Consumption Indication Erroneous

Discussion in 'Gen 2 Prius Audio and Electronics' started by MontyB, Nov 8, 2016.

  1. MontyB

    MontyB Junior Member

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    Good morning from Michigan:

    I am the owner of a 2008 Prius that I purchased used with 86,000 miles on it. It's been a great car and has surpassed my expectations.

    Recently I have observed that the "average fuel consumption" indication per miles driven since the "reset" does not correspond with what the "instantaneous fuel consumption" indicates.

    For instance, this morning I filled up and CLOSELY observed my instantaneous fuel consumption indication as I drove the first 40 miles of that respective tank of gas (since "reset"). This was easy to do as I set my cruise control at 52 mph on a county road and, according to the instantaneous fuel consumption indication was in the 50-56 mph hour range (or more!) for probably 99% of those first 40 miles. The problem was (is), at the conclusion of those first 40 miles my "average fuel consumption" was indicated to be 44.1 mpg.

    In hindsight, for the past few thousand miles my "average fuel consumption" has consistently been reported as 44.1 or 44.2 mpg. Previous to the last few thousand miles my "average fuel consumption" per tankful had always been in the 47-48 mpg range. At first I had attributed this drop in fuel economy to the colder weather (although for Michigan we have been experiencing a VERY mild fall). However, after paying CLOSE attention to my "instantaneous fuel economy" indication under situations when it has been easy to compare my "instantaneous" fuel economy to my computed "average" fuel economy for a specified distance, my "average" is well below where it should be.

    Again, this morning, other than the first 1/8th of a mile that it took me to get up to 52 mph for the remaining 39 7/8th miles (driven with the cruise control set) the instantaneous fuel consumption reading NEVER dipped below 50 mpg, yet at the end of 40 miles the "average" indicated 44.1 mpg for those 40 miles.

    Anyone else experience anything like this?

    Have a good day all. Thanks in advance for your valued feedback.
     
    #1 MontyB, Nov 8, 2016
  2. bisco

    bisco cookie crumbler

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    the best method for tracking fuel consumption is manually calculating it when you fill up, and keeping a running average.
    prius gauge reporting is iffy at best. if you feel that your mpg's are going down, there is a great sticky questionnaire you can fill out and post. there are so many things that can cause reduced fuel economy, a lot of testing and guesswork is involved. but answering the questions ahead of time can speed up the process. all the best!(y)
     
    #2 bisco, Nov 8, 2016
  3. Mendel Leisk

    Mendel Leisk EGR Fanatic

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    Trying to judge what average mpg should be, based on instant mpg, is pretty much impossible. Instant mpg dances around with every touch of the gas pedal.

    The best I can say about the car's displayed average mpg, it's inaccurate but with a fairly predictable/consistent positive bias, ie: you can take what it's saying, factor in the fudge percentage it's adding, and have good odds of knowing the mileage as hand calculated.
     
    #3 Mendel Leisk, Nov 8, 2016
  4. fuzzy1

    fuzzy1 Senior Member

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    Because of the specific math involved, you cannot average MPGs over time or distance. The 0 MPG time while starting and standing still pulls the overall MPG down far more than any 100 MPG gliding pulls it up.

    It would be much easier to visually or mentally 'average' if we used the Euro-style consumption measure, liters per 100km, or its non-metric equivalent,gallons per 10 miles. The intuitive results would then more closely match the mathematical reality. But Americans resist,
     
    #4 fuzzy1, Nov 8, 2016
  5. MontyB

    MontyB Junior Member

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    I understand that, except that previously driving the same 40 mile stretch of road at the same cruise control governed speed of 52 mph...the "average" fuel consumption more closely mirrored that of the reported instantaneous fuel consumption.

    I've driven this car for nearly 13,000 miles and I am very familiar with it's "typical" fuel economy. Previously getting an instantaneous 50-56 mpg over a specific distance did not result in the computed average reflecting 10 mpg less than the instantaneous average.
     
    #5 MontyB, Nov 8, 2016
  6. dolj

    dolj Senior Member

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    One thing I'm not sure of from what you have said is are pressing the "Reset" button on the "Consumption" screen at every tank refill, or are you relying on the auto reset feature?

    If you are relying on the auto rest feature, this behaves in a different way on the early Gen II (2004-2005) compared to the face lift model (2006-2009).
    • 2004-2005: Auto resets the distance and average consumption readouts.
    • 2006-2009: Auto resets only the distance readout.
    On the 2006-2009 Gen II (such as the OPs), unless the OP reset the average consumption manually, it calculates the average from the last time it was reset. If the OP has never reset this in the 13,000 miles travelled, then the average is calculated over that 13,000 miles he has clocked up. Over such a large distance, one is not going to see much movement from 44.1 mpg even if one averaged 52 mpg over a whole tank, let alone 40 miles.

    I agree with the other comments above, that if you want to track consumption accurately, you need to record somehow and manually calculate. Using the website fuelly,com is another easy option.

    I don't know if this helps, but hope it does.
     
    #6 dolj, Nov 8, 2016
  7. bisco

    bisco cookie crumbler

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    it's probably the weather an reformulated gasoline, but check your tyres pressure, battery and oil level.
     
    #7 bisco, Nov 8, 2016
  8. MontyB

    MontyB Junior Member

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    Dolj,

    Thank you so much for sharing your knowledge of the distinction between what the auto reset feature does for the '05-'06 models compared to what occurs on the '06-'09 models. Your explanation would seem to account for why the average fuel consumption never seems to vary much. I typically will go long distances (several fill ups) without manually hitting the "reset" feature on my '08 model. It would make sense that if the average has been calculated based on the previous several thousand miles of driving rather than the distance driven since the last fill up (just 40 miles in this example), that the average fuel consumption would not be significantly impacted.

    Have a great day all.

    -Monty in Michigan.
     
    #8 MontyB, Nov 9, 2016
    bisco likes this.
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