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Fuel/MPG discrepancy in 09 Prius

Discussion in 'Gen 2 Prius Care, Maintenance and Troubleshooting' started by adllewellyn, Oct 13, 2010.

  1. adllewellyn

    adllewellyn New Member

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    I have read various threads about this issue but can't seem to get an official opinion about whether this is an issue that necessitates service. I am finding a big discrepancy between what my display tells me I am getting in mileage and what I can calculate by hand.

    Last five tanks:
    9.4 gal in, 50.1 mpg, 466 mi - 470 by hand
    9.0 gal in, 48.4 mpg, 459 mi - 435 by hand
    9.1 gal in, 44.9 mpg, 386 mi - 408 by hand
    7.5 gal in, 46.9 mpg, 402 mi - 351 by hand
    9.8 gal in, 51.1 mpg, 395 mi - 499 by hand

    All tanks were refilled as my Prius went to one dot left. And I know I don't get great mileage - I live where it is fairly flat and tend to take short trips. Can't figure out why it only took 7.5 that tank either. Sigh. I'm frustrated.

    As you see, the discrepancy is quickly getting worse. Off by 4, off by 24, off by 24, off by -51(?), off by almost 100? I know this problem has been seen before, and there are different variables, but this seems excessive.

    Thoughts? Please help, Prius-geniuses. :)
     
    #1 adllewellyn, Oct 13, 2010
  2. bisco

    bisco cookie crumbler

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    i don't follow you. is there a descrepency between you're calculated mpg and display mpg? or a descrency between odometer and what, i don't know? how can you calculate distance manually?
     
    #2 bisco, Oct 13, 2010
  3. vertex

    vertex Active Member

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    He is stating the number of miles he should have gone for the gas added and the reported MPG, vs. the miles he actuall went.

    For all the trips, your average MPG by the MFD is 47, and by the actual fuel consumed is 47.053, so you are doing really well! The difference is due to the well known bladder issue, where the size of the tank is changing due to the bladder.
     
    #3 vertex, Oct 13, 2010
    1 person likes this.
  4. adllewellyn

    adllewellyn New Member

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    Sorry for being unclear.
    Yes, there is a discrepancy between what my Prius tells me I am getting for mileage/MPG (the first three numbers) and what I calculate (the last number.)

    I check MPG manually by multiplying my average MPG for that tank by the number of gallons I have just replaced. (which in my reading of other threads seems to be a common practice.)

    I do realize that this method is imprecise - bladder, temperature, other variables will cause differences. But I would expect it to be somewhat close - or at least to have a relatively consistent "margin of error." But to be getting such wildly different numbers is startling to me; 5-10 miles off per tank would be one thing (although I would have expected better from Toyota.) But to go from -50 to +100 off? That seems like a problem.

    Just trying to see if anyone else has seen this issue manifest in this way or has any ideas?
     
    #4 adllewellyn, Oct 13, 2010
  5. derkraut

    derkraut Member

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    Yup....nothing out of the ordinary here, imho. The fuel tank bladder is what causes the discrepancies. I certainly wouldn't worry about it.:)
     
    #5 derkraut, Oct 13, 2010
  6. adllewellyn

    adllewellyn New Member

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    all right... this is what i'm trying to find out... thanks for the input. :)
     
    #6 adllewellyn, Oct 13, 2010
  7. Patrick Wong

    Patrick Wong DIY Enthusiast

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    This is not an issue that requires service.

    The root cause of the problem is three-fold: 1) The presence of the bladder within the fuel tank means that the tank's capacity is variable, 2) The fuel gauge is not particularly accurate, so the amount of fuel that you have left in the tank when the gauge shows one bar may vary from one refueling to the next, 3) The amount of fuel that you can pump before the fuel pump shuts off is variable, depending upon the pump.

    Therefore, if you wish to track mpg, you need to log cumulative miles driven and cumulative gallons pumped. Over several refuelings, individual errors will tend to cancel out.

    As previously stated, your average fuel economy over the sample data that you provided is 47 mpg, which is quite good.
     
    #7 Patrick Wong, Oct 13, 2010
  8. Danny Hamilton

    Danny Hamilton Active Member

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    The confusion comes from assuming that the amount of fuel you add to the tank is an indication of the amount of fuel you burned,

    There are a few reasons that this can often be a false assumption, some of which (IIRC) are spelled out in the owners manual.

    Two of the more significant reasons are that the volume of fuel that the bladder in the tank can hold varies over time (due to factors such as temperature and age), and the automatic shut-off on the pump can cut off earlier or later on any fill-up

    Looking at the numbers you provided, if we assume that the odometer and the MPG reported by the prius are correct, the last 5 tanks could work out as follows:

    Perhaps 10.7 gallons in tank to start?

    drive 466 miles at 50.1 MPG = 9.3014 gallons burned, 1.3986 gallons remaining, 9.4 gallons added, 10.7986 gallons in tank

    drive 459 miles at 48.4 MPG = 9.4835 gallons burned, 1.3151 gallons remaining, 9.0 gallons added, 10.3151 gallons in tank

    drive 386 miles at 44.9 MPG = 8.5969 gallons burned, 1.7182 gallons remaining, 9.1 gallons added, 10.8182 gallons in tank

    drive 402 miles at 46.9 MPG = 8.5714 gallons burned, 2.2468 gallons remaining, 7.5 gallons added, 9.7468 gallons in tank

    drive 395 miles at 51.1 MPG = 7.7299 gallons burned, 2.0169 gallons remaining, 9.8 gallons added, 11.8169 gallons in tank
    ​
    Notice that the discrepancy you are calculating is because of the difference between fuel burned and fuel added and perhaps a bit of an effect from rounding the fuel added number to the nearest tenth of a gallon (which at the MPG you are working with can effect the outcome of your math by between 4.5 and 5.1 miles)

    9.4 - 9.3014 = +0.0986 gallons at 50.1 MPG = +4.9399 miles
    9.0 - 9.4835 = -0.4835 gallons at 48.4 MPG = -23.4014 miles
    9.1 - 8.5969 = +0.5031 gallons at 44.9 MPG = +22.5892 miles
    7.5 - 8.5714 = -1.0714 gallons at 46.9 MPG = -50.2387 miles
    9.8 - 7.7299 = +2.0701 gallons at 51.1 MPG = +105.7821 miles
     
    #8 Danny Hamilton, Oct 14, 2010
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