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Odd MPG Results

Discussion in 'Gen 2 Prius Fuel Economy' started by sdgeiger, Apr 17, 2005.

  1. sdgeiger

    sdgeiger Junior Member

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    Yesterday I was down to a single flashing bar, but yet, when I filled the tank up, I was only able to pump in 8.632 gallons, this was after reaching automatic shutoff three times.

    My total miles was 484, which computes to 56.07 MPG for the tank of gas, but yet the MFD showed 49.8.

    This is really annoying. I'm sure I didn't get 56.07 MPG. I suppose the pump was incorrect, but I can't be sure.

    Has anyone else experienced this?
     
    #1 sdgeiger, Apr 17, 2005
  2. hdrygas

    hdrygas New Member

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    Yes I have seen it several times. I now suspect the calculated tank averages are in accurate because of all the factors mentioned before. I had a really notable one with "my best tank ever" a month ago. I suspect it was a constellation of cold weather so the bladder was stiff, a really sensitive pump that I had a lot of trouble filling the tank and a not so empty tank to begin with. So I get 57+ mpg. It caught up with me. I think the only calculated number that is any good is the life time total miles and total gallons used. I notice that the average calculated and a average of the tank mpg on the MFD ore slowely converging. I think that for one tank that the MFD is closer to truth and tends to under report mpg by a small amount. We shall see what summer brings.
     
    #2 hdrygas, Apr 19, 2005
  3. sdgeiger

    sdgeiger Junior Member

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    Today I filled my tank up again. This time I continued to pump until I couldn't get any more into the tank.

    My results:

    433 Miles
    Gallons-10.864 (**Has anyone ever gotten that much gas into their car?)
    Calculated MPG - 39.86
    MFD - 49.7


    If I take an average over the last two tanks, the calculated MPG is 47.04, which seems about right. The only thing I can think of that could have caused the problem during the previous tank is that there was a malfunction with the pump, which prevented me from putting enough gas into the car.
     
    #3 sdgeiger, Apr 24, 2005
  4. Prolene

    Prolene New Member

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    My BFD tends to suggest better mpg by about 4 mpg per tank compared to what I calculate.
     
    #4 Prolene, Apr 24, 2005
  5. Ray Moore

    Ray Moore Active Member

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    I have added as much as 12.75 on my 763 mile tank and over 11 on almost every tank. Several have been over 12 gallons. I usually stop for gas at around 650 miles unless I spend too much time on the highway on long trips at 70 or 80 mph. At those speeds the mileage plummets to 50 or even 40 MPG.
     
    #5 Ray Moore, Apr 24, 2005
  6. coloradospringsprius

    coloradospringsprius Member

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    Any given tank may be way off. From the Gas/Fuel FAQs at http://www.vfaq.net/:

    "The seal on the filler tube plate can cause the backflow sensor in the pump's filler to click off well before the tank is truly full."

    The mileage will even out over time, but for any particular tank, the screen calculation is probably more accurate. I've had discrepancies as much as 8 mpg lower than the screen calculation and 26 (!) mpg higher.
     
    #6 coloradospringsprius, Apr 24, 2005
  7. sdgeiger

    sdgeiger Junior Member

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    <div class='quotetop'>QUOTE(Ray Moore\";p=\"83982)</div>
    Ray,

    Now I'm really confused. Per page 313 of my owners manual, the Prius's fuel capacity is 11.9 gallons. So how did you get 12.75 in there?

    -Scott
     
    #7 sdgeiger, Apr 24, 2005
  8. Ray Moore

    Ray Moore Active Member

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    Yeah, the owners' manual also says 3.9 quarts of oil but of course that will always result in an overfill. If you carefully fill the Prius up to the top of the filler tube, you can get in around 13 total gallons. Some here will warn against it and there have been somewhat lengthy discussions on the practice. I believe that there is a valve that prevents the practice from flooding the canister and my mileage results prove that it will not reduce your MPGs. YMMV
     
    #8 Ray Moore, Apr 24, 2005
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