Will there ever be a fix for the MPG estimates?

Discussion in 'Gen 3 Prius Fuel Economy' started by s1njin, Jan 11, 2011.

  1. eldiee

    eldiee Member

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    This subject has been gone over in several threads last year and especially in 2009 when the 2010 Prius was newly released. I belive that they have left it reading higher than what most of us get in actual measurement on purpose. It makes the average operator that is not into detailed records feel good. Thus they can say positive things to their friends about their prius. In a span of 18 months ours has varied from 3% to 6% higher than actual. I can not complain as our lifetime average is still over 52mpg. The computer reading 55.1 since we got the 2010 Prius
     
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  2. HTMLSpinnr

    HTMLSpinnr Super Moderator
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    I carefully track "real" (miles vs gallons filled) vs. display in a Google Docs spreadsheet linked in my signature (MPG Tracking).

    If you want to add one more "opinion" to the mix, try letting a ScanGauge track it. Reason: Your speedometer reads slightly high (they're allowed to), thus your odometer also reads slightly high. ScanGauge compensates for this and, based on fuel flow calculations (on a properly calibrated SG), comes up with potentially even more accurate results - typically lower than using miles divided by gallons. The percentage increases as your tires wear and "resets" upon installing new tires. Even different tire models of the same size can contribute somewhat.

    What I'm suggesting is that the 2-3% "over" error in speedometer/odometer may contribute to the 5% or more MPG discrepancy of the gauge.

    Thoughts?
     
  3. fuzzy1

    fuzzy1 Senior Member

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    Displayed speedometer and odometer errors should be different, and mine are. While my speedo reads slightly high, my odo reads a fraction of a percent low (0.2-0.3%), or at least it did when the tires were new.

    Speedometers are biased high due to various government regulations, something about not giving drivers an excuse for speeding.

    Odometers are (or soon will be) biased low due to class action lawsuits about warranty fraud, such as those that hit my previous car brands (Honda and Subaru). I believe both settled by giving a 2% odometer extension on the warranties.
     
  4. Mendel Leisk

    Mendel Leisk Sand Pounder

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    Regardless of odometer accuracy, couldn't the displayed mpg, presumably calculated with the displayed mileage, more closely mirror calculated mpg, also calculated with the diplayed mileage.

    Geesh, I'm not sure if I understand that myself, getting tired ;)
     
  5. fuzzy1

    fuzzy1 Senior Member

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    ^^ I believe Gen2 proved that it could.
     
  6. bac

    bac Active Member

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    The federal government sets these numbers - not Toyota or any auto maker. They use the exact same parameters when measuring fuel economy in every auto.

    So - don't take ANY auto's predicted mpg seriously - there are way too many variables! This number is really for comparisons only - and it works great in its designed capacity.

    As already stated, your mileage may (will!) vary.

    -Brad
     
  7. Joe166

    Joe166 New Member

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    With the possibility of out of spec pumps (which might be the least likely since someone at least checks them once in a while and there is a fine possible), and the obvious problems with always filling the tank to the exact same place, who is so sure that the personally computed numbers are any more reliable than the car computed ones?

    Besides, what possible difference could it make? It is only a number that really doesn't matter, does it?
     
  8. Mendel Leisk

    Mendel Leisk Sand Pounder

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    Our Honda Civic Hybrid underestimated mpg. If it said 5.0 liter/100km, calculated would be 4.8~4.9. Very consistantly.

    Our Prius, in the few tanks I've calculated, consistantly overestimates: if it said 5.0, calculated is 5.3~5.4. Roughly double the inaccuracy, to the optimistic side.

    I'm sure a patch could fix this.
     
  9. Teakwood

    Teakwood Member

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    MPG gauges have inherent inaccuracies.
    Get over it.
     
  10. twittel

    twittel Senior Member

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    I forever a pessimist, believing that Toyota intentionally allows MPG's to reader slightly higher than actual. Of course, the only real way to calculate MPG is to do it manually and even that may be skewed by pump error.

    The other possibility for MPG errors may be differences in fuel mixtures from the pump's supplier; i.e. ethanol percentages, water, octane inconsistencies, etc. I would think these issues could influence actual MPG readings across multiple fill-ups. Last but not least, let's not discount warm/cold engine consumption, road contitions, blah, blah, blah...


    Happy Motoring,

    Tom
     
  11. fuzzy1

    fuzzy1 Senior Member

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    Yes, this much error in the pump is a violation state weights and measures laws
    This is a problem only for single refills. When combining fifty consecutive tanks, the probable error becomes tiny.
    Having checked the odometer too, I am quite sure.
    I'll guess that you are not an engineer or accountant.
     
  12. robbyr2

    robbyr2 New Member

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    The EPA figures seem correct for my V. I was averaging 54 mpg until the recent blast of cold weather reduced it to about 51 mpg. At the pump.
     
  13. kneedraggin

    kneedraggin Junior Member

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    You are driving one of the most fuel efficient cars in the WORLD...
    why are you even letting the few mpg inaccuracy bother you.

    just my .02
     
  14. jdcollins5

    jdcollins5 Senior Member

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    My difference between displayed MPG and calculated has been a steady average of 3.3 mpg for about 18 months now. It is up and down between tank refills but the average has remained fairly consistent.

    I really would like for Toyota to fix this issue. I automatically subtract the 3.3 mpg from the display number but would much prefer for the display to be reasonably accurate.
     
  15. xs650

    xs650 Senior Member

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    Some of us don't appreciate Toyota intentionally having a 5% optimistic error in the system. an error that they could easily take out.
     
  16. alfon

    alfon Senior Member

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    I agree Toyota should correct this discrepancy.

    However, I believe short fillups, say under 150 miles, may be more
    consistently accurate.

    alfon
     
  17. jhinsc

    jhinsc Senior Member

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    Every vehicle I've owned that had a computer mpg indicator has always been off to the high side. It's seems more so in the Prius because a 6% difference is numerically higher (3.5 mpg difference) due to it's high mpg rating than a vehicle rated at 20 mpg (1.2 mpg difference). What is reasonable accurate for you?
     
  18. rebenson

    rebenson Member

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    not that much of a big deal, though would like adjustment to miles to empty... I have never put close to 10 gallons in the tank, including a 530 mile trex
     
  19. jdcollins5

    jdcollins5 Senior Member

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    Reasonably accurate to me would be +/- 1 MPG.

    From posts by the Gen II owners, the Gen II seemed to be much closer than the Gen III. Within what I would consider reasonable. This was apparrently a marketing ploy by Toyota.
     
  20. Skoorbmax

    Skoorbmax Senior Member

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    Mine seems about 5%, really surprising actually that such a huge error in this direction is noted by all of us. They simply calibrated the calculations incorrectly.
     
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