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MPG Computer Fuel Consumption Gauge Accuracy

Discussion in 'Gen III 2010+ Prius Fuel Economy' started by djfnw, Apr 18, 2011.

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  1. djfnw

    djfnw Junior Member

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    I have a 2011 Prius III. When I fill the gas tank it shows ten pips. The tank is 11.9 gallons so each pip represents 1.19 gallons. I’m manually calculating my gas mileage to compare it to the mpg consumption gauge.

    The first pip disappeared this morning at 49.4 miles. The consumption gauge says 46.8 mpg for those 49.4 miles. The manual calculation comes to 41.5 mpg. I know it’s a small sample size and I’m going to keep monitoring it.

    Is the mpg consumption gauge accurate?
  2. krelborne

    krelborne New Member

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    Wow, measuring down to the pip? The pips seem to fall pretty reliably, but there's surely a lot of error doing that. Most people report an optimistic consumption estimate from the car. Mine has always been at least 2 MPG more than the actual calculated mileage, usually more like 3-4 MPG.
  3. ETC(SS)

    ETC(SS) Aspiring Hypocommuter.

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    The MDF is off by about +5% (reads about 5% high as opposed to hand calculating tank-to-tank.) Your theory that each pip represents 1.19 gallons may be a little off. This would presume that the fuel level gauge is linear throughout your tank range---and I've ever managed to pump more than 10-gallons of fuel into my Prius no matter how long my testicular fortitude allows me to drive after the last pip starts flashing---which to date has been about 50 miles.
    I don't know what shape the gas tank is, or what kind of daffy formula that the engineers came up with in Aichi to calcuguess how many millimeters of float travel equals how many liters of fuel, but my assumption is that the pips aren't calibrated to be exactly 1.1900 gallons for each, no matter how they compensate for platform lean, temperature, sloshing, etc.

    I usually calcuguess my miles-per-gallon by the tank...at the pump...and with a pencil. :D
  4. fuzzy1

    fuzzy1 Senior Member

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    No they don't.

    Did your previous cars' fuel tanks run dry at the same moment the fuel gauges reached 'E'? Not mine. Most had 2 to 4 gallons of their claimed capacity hidden away below 'E'. The one exception went dry with the needle still above 'E'.

    Bob Wilson tested the Prius for us, and found about 2 gallons below the flashing pip. That means each pip averages closer to 1.0 gallon, not counting the nonlinearities that cause each pip to represent a different amount. And my ScanGauge reveals a lot of sloshing around in the tank. Like my previous cars, the Prius fuel gauge only partially filters out that slosh, which is why the gauge sometimes climbs a bar or two, depending on the slopes and corners of the past few miles.



    Not exactly, but it is much more accurate than the fuel gauge.
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  5. DaYooper

    DaYooper New Member

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    I am also thinking that since the standard mph is 1.5% to fast the actual miles driven is less than what is shown.

    Sent from my SCH-I500 using Tapatalk
  6. fuzzy1

    fuzzy1 Senior Member

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    The former does not cause the latter.

    Speedometer displays are biased high for legal and industry standard reasons, so that drivers cannot use the inevitable errors as an excuse for speeding. The ECU has an internal unbiased speed reading, which you can display with a ScanGauge or other engine monitor.

    Odometers cannot be similarly biased without inviting class action lawsuits for warranty fraud. Just ask Honda and Subaru. My Prius odometer read about 0.2% low when the tires were young.
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  7. DaYooper

    DaYooper New Member

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    That is interesting. So the ECU figures mph seperately from distance traveled. That means that the fuel consumption indicator must be closer to actual consumption, but the average mph reading is still off.
  8. alfon

    alfon Active Member

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    Remember your 2010 Prius will still
    hold two more gallons of gas after
    the pump first clicks off.

    This is filling to the top of the filler neck.

    alfon
  9. xs650

    xs650 Senior Member

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    That depends on how good the cut off on the pump nozzle is and how fast you were pumping. Mine fills to the neck in under a gallon when I use a well maintained gas station.
  10. djfnw

    djfnw Junior Member

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    If each pip is approx 1 gallon then I am averaging 45 miles per pip which is pretty close to the computer calculated mpg.
  11. qbee42

    qbee42 My other car is a boat

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    Speed and distance are separately measured. International treaty and US laws govern both. Generally speed readings are biased a bit fast, while distance (odometer) readings are required to be accurate.

    Tom
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