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Newbie MFD Question

Discussion in 'Gen 2 Prius Main Forum' started by tadyergey, May 25, 2006.

  1. tadyergey

    tadyergey Junior Member

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    Hey all, I looked through other posts for this answer and had no luck finding it. Please forgive if this is a repeat.

    Does the MFD internally calculate MPGs above 99.9 and only have a presentation limitatiion or is it capped mathmatically also?

    Thanks in advance!

    Tad
     
  2. AnOldHouse

    AnOldHouse Member

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    <div class='quotetop'>QUOTE(Tad @ May 25 2006, 10:59 AM) [snapback]260883[/snapback]</div>
    You are correct.

    Note that the MFD calculations are usually a bit more optimistic than reality, by about 2-3 mpg in my experience.
     
  3. viking31

    viking31 Member

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    <div class='quotetop'>QUOTE(Tad @ May 25 2006, 10:59 AM) [snapback]260883[/snapback]</div>
    I believe it is just a display limitation. If it wasn't the mpg readout would be lower than actual.

    I too, however, have noticed the MFD display is usually about 2-3 mpg's above my actual calculated figures. I assume it is related to the precision limitation of the flowmeter (or whatever device is used to calculate the real time usage of fuel) OR for the conspiracy theorists it is actually "fudged" a little high ;-).

    Rick
    2006 #4
    West Central FL
     
  4. brandon

    brandon Member

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    Of course, we're assuming that your calculations reflect the "actual" data. How do you calculate gas usage? In other words, how do you know how much gas has been used? If the capacity of the fuel bladder is in constant flux, how does one know when a gallon of fuel has been used?
     
  5. tadyergey

    tadyergey Junior Member

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    Thanks for the replies.

    I have found in my vast experience (4 tanks) that the MFD and my Automobil program on my Palm PDA are only about 0.1 mpg off lifetime, however the MFD and the old fashioned miles/gallons pumped have differed by up to +/- 4.5 mpg by each tank. Regardless it beats the 22 mpg of my old RAV4!

    Thanks again!

    Tad
     
  6. Charles Suitt

    Charles Suitt Senior Member

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    :mellow: I agree with a previous post... the difference between the MFD and calculated MPGs runs 2-4 gallons per mile on a given tank. Overall, I find the MFD to be over 90% accurate.

    The MFD calculations are based on the number of times a fuel injector fires (how much fuel is injected) versus the distance traveled. The calculated MPGs is based on the distance traveled versus the gallons pumped which can be variable due to the fuel bladder peculiarities.

    I prefer to use the calculated MPGs over an extended period - in my case 18 months. In either case, my fuel economy is a bit better than twice that of our '96 4-cyl Camry.
     
  7. qbee42

    qbee42 My other car is a boat

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    And you have to assume the pumps at the gas station aren't fudging the volume figures. I can't imagine they would ever do that. :rolleyes:

    Tom
     
  8. darelldd

    darelldd Prius is our Gas Guzzler

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    <div class='quotetop'>QUOTE(brandon @ May 25 2006, 10:31 AM) [snapback]260942[/snapback]</div>
    That's really not a problem, is it? Assuming you can trust the numbers on the pump. You simply total up every drop of gas that has gone into the car, and divide by the total of miles that your car has rolled. With enough fillups and enough miles, you're going to be pretty darn accurate. Trying to do it for each tank is foolhardy at best.
     
  9. brandon

    brandon Member

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    Well, my thought is that it won't be any more accurate over a period of time than it would be calculated per tank... you'll just have more of the same inaccurate numbers to work with.

    *EDIT* Except I suppose it might be slightly more accurate if you use those totals like you mentioned rather than average the individual calculations from each fillup like I was thinking... the wheels in my head may be turning slowly tonight, but they are turning... :rolleyes:
     
  10. qbee42

    qbee42 My other car is a boat

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    <div class='quotetop'>QUOTE(darelldd @ May 25 2006, 11:37 PM) [snapback]261178[/snapback]</div>
    Darell is correct. If you total the fuel and the miles, the percentage of error introduced by filling uncertainty is reduced. The total error remains the same, but this total error is now distributed over a larger number of gallons, so accuracy is improved.

    Another way to think of it is like this:

    1) You fill the tank the first time and record the volume of the fill. You know how much was added, but you are not really sure how full the tank was to begin with. For the sake of an easy discussion, let's say the amount of uncertainty is 1 gal from a 10 gal tank. This mean we potentailly have a 10% error.

    2) Drive around and burn all of the gas. Now we fill the tank again. We still don't know how full the tank was or is when the pump clicks off, but we still know it is within the 1 gal error range (for the sake of this discussion). Now we have a 1 gal error over 20 gals of fuel, which is a 5% error.

    The error percentage trails off asymtotically towards zero.

    This assumes, of course, that your data is good. If the pump or odometer are giving you false numbers, then you have another source of error.

    Tom