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displayed mpg 2% higher than actual

Discussion in 'Gen 2 Prius Fuel Economy' started by mthiker, Jun 5, 2012.

  1. mthiker

    mthiker New Member

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    mthiker New Member

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    After tracking my fuel usage very carefully for the first two and a half years that I owned my 07 Prius II (recording gallons pumped, miles driven and displayed mpg for that tank and then resetting the car's trip odometer and mpg calculator, at every fill-up). Over that time period the average of the displayed MPG was 49.1 mpg, and of the average of the hand-calculated MPG was 48.0 mpg; this is a 2.2% (i.e. about 1 mpg) overestimation of displayed fuel efficiency. Such a systematic error must be due to some combination of car computer overestimation of miles driven ( e.g. due tire diameter smaller than assumed--all aftermarket tires I've bought recently are about 2% undersized), and/or underestimation of the amount of fuel inject per inject. Or, since gas station pump meters cannot be perfectly accurate, maybe they tend to be calibrated to overestimate the amount of gas pumped (its probably safe to assume that a station owner would tend to err in his favor, if he was going to err). Whatever the error source, since it is systematic, Toyota should be able to easily correct for it, and in fact it would be nice if the owner could enter a wheel diameter calibration factor, such as is used for a bicycle speedometer/odometer.​

    Read more: http://priuschat.com/threads/displayed-versus-computed-mpg-2-high.110310/#ixzz1wxE3gfi5
     
    #1 mthiker, Jun 5, 2012
  2. tdelker

    tdelker Junior Member

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    Tire diameter can't be the problem unless you aren't using your odometer to calculate the mileage. Maybe you are using google earth? That's not very accurate...

    I suspect that it's the flow rate of the injectors that is slightly out.

    Tom
     
    #2 tdelker, Jun 5, 2012
  3. F8L

    F8L Protecting Habitat & AG Lands

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    Or maybe Toyota designed it that way. The GenII is generally believed to overestimate by 2% and the GenIII overestimates by approx. 5%-6%.
     
    #3 F8L, Jun 5, 2012
  4. tdelker

    tdelker Junior Member

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    I doubt it is a conspiracy. I think it's just a difference between the design and the actual built injector flow rate, but that's admittedly just a guess.
     
    #4 tdelker, Jun 6, 2012
  5. Mendel Leisk

    Mendel Leisk Senior Member

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    My 3rd gen error bounces around between 6 and 8 percent, always optimistic. And fwiw, our previous Honda Civic Hybrid (2nd gen) was very consistantly around 1% pessimistic.

    I don't think it's due to mechanical issues beyond Toyota's control: the error is quite gross, and always in favour of Toyota.
     
    #5 Mendel Leisk, Jun 6, 2012
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