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2016 Odometer Error

Discussion in 'Gen 4 Prius Fuel Economy' started by krousdb, Apr 8, 2016.

  1. JohnF

    JohnF Active Member

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    Mendel, more than some people seem to think. The statement "to expect averaging to make variables irrelevant is a misunderstanding of statistics." is in itself a misunderstanding of statistics, not to mention the scientific process.

    It seems best to view the Prius' MFD numbers as entertainment. As fuzzy1 points out, there is no legal pressure for them to be accurate, whereas gallons pumped are supposedly certified to within 0.5% in many states, an acceptable limit given the difficulty of measuring to greater accuracy. As I said, the fact that the Gen 3 Prius' MFD numbers invariably overstate the MPG versus ODO/gallons pumped by a consistent percentage whereas the Insight 's MFD did not convinces me that something fishy with the Prius' methodology is introducing a systematic error.

    Thanks to GPS it's easy to check odometer accuracy and correct that to a reasonable level of precision.

    Unless there is input from the moderators, I'll determine MPG as gallons pumped divided by GPS-corrected ODO reading, as Wayne Gerdes does. I'll probably gather GPS data on several runs to see how consistent the correction is. If it is consistent, I'll just use an average correction factor for the ODO reading and recheck it from time to time.

    I suppose sloppy numbers can be used "for reference" to compare to other sloppy numbers determined in the same way, but they don't have much value in comparisons to the real world. Ironically, we have a government bureau, the NIST, devoted to accuracy in measurements, and they use "reference standards" as the bedrock against which methods are calibrated.
     
    #41 JohnF, Apr 13, 2016
    Last edited: Apr 13, 2016
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  2. Mendel Leisk

    Mendel Leisk Senior Member

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    I think the Toyota engineers are succumbing to internal pressures, the numbers are invariably and consistently at the edge of credulity, lol. It's so obvious as to be an embarrassment.

    And, I don't really buy into the thesis, that inaccuracies and variables compound: my experience is the longer you keep records the more accurate, and unbudgeable, the results.

    With close to 120 fill ups calculated, I'm seeing an unwavering 7.5% positive bias, minor oscillations to either side not withstanding.

    Meh, should have held fast on "what do I know".
     
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  3. fuzzy1

    fuzzy1 Senior Member

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    I also seem to remember other comments about the Civic Hybrid being closer, even low, and the Gen2 Prius being closer than the Gen3.
     
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  4. Mendel Leisk

    Mendel Leisk Senior Member

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    Yup, our 06 Civic Hybrid was either spot-on, or low by 0.1, in the liters/100km system. It ain't hard.

    Of course, the numbers were more likely be 6 point something, vs 4 point something with the Prius, but at least they were honest, lol.
     
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  5. JohnF

    JohnF Active Member

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    I found even the estimated 5% error on my 2011 Gen 3 to be extremely annoying. If I had good conditions and got, say, 70mpg on the MFD, I'd be really pleased until I realized it was probably more like 66.5mpg. Phooey!

    At least subtracting 5% is easy: divide by 10, take half of that, and subtract. None of those gymnastics with the Insight-I.
     
  6. Mendel Leisk

    Mendel Leisk Senior Member

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    If our display is (for example) 4.5 liters per 100 km, calculated is invariably 4.8 or 4.9. Very easy accommodation, but yeah: frustrating. It's like that hyperbolic acquaintance... :rolleyes:
     
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  7. krousdb

    krousdb NX-74205

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    I don't remember my Gen 3 being off by 7.5%. It was closer to 5% like my Gen 2. I am wondering if the L/100km format and the lack of a third significant digit introduces the additional error. The fact that improving your FE increases your error (then number gets smaller rather than larger) could be the culprit. But even so you would think it would average out in the long term.
     
  8. Mendel Leisk

    Mendel Leisk Senior Member

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    I'm not sure, but I would suspect if my fuel economy increased significantly, the .3~.4 difference would become .2~.3. In short: the percent would hold, not that arithmetic difference.
     
  9. krousdb

    krousdb NX-74205

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    I used the Speed Tracker app today to track my commute. It has a few nice features. When I got to work, I paused it. On my way home I resumed it. It also measures time while stopped so the average speed excludes waiting at lights. It gives a much better indication of average speed for the trip by taking out the stop light variable.

    127.3 miles shown on Speed Tracker. 125.0 miles shown on the Eco Diary. That is a 1.84% under-registration of the Odometer with Bridgestone Ecopia 422+ tires. This is consistent with the other testing done earlier. I also double checked the triple 17 mile loop and duplicated the results. 50.0 miles indicated, 51.0 miles shown on Speed Tracker, or 2.0% under-registration.

    Another note of interest. This is my first trip with E10. My previous best RT was 90.5 MPG with E0. The 87.3MPG trip with E10 represents a 3.7% difference, which is close to the 3% expected result. Too many variables to draw any conclusions from just one trip but it is the first data point to on a Gen 4 as far as know.

    Capture.JPG Captureq.JPG
     
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  10. JohnF

    JohnF Active Member

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    Thanks for doing this. It seems that SpeedTracker is accurate enough for our purposes so we don't have to resort to more painstaking methods to measure actual mileage.

    I have Toyo tires and so far have done three calibration runs using SpeedTracker, as noted previously:
    I have to do a few more runs to pin it down more closely. You are finding a larger error, perhaps car-to-car variation or perhaps a difference in the tires Toyo vs Bridgestone.

    Incidentally, SpeedTracker has another screen that displays MPH, which can be used to check the speedo if one cares.
     
  11. krousdb

    krousdb NX-74205

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    I believe that the difference is due to the difference in revs per mile for each tire. I finally tracked down the revs per mile specs for the EP 422+, both 15" and 17" here:
    Bridgestone Ecopia EP422 Plus Tires

    Mine are the P195/65R15, 1279 load, 44psi, 17 lb weight, 831 revs per mile. The touring models have the 215/45R17, 1201 load, 51psi, 20 lb weight, 841 or 843 revs per mile. That is about a 1.4% difference and would put the touring model very close to matching with the odometer.

    Unfortunately the Toyos and the Dunlops are not listed.
     
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  12. krousdb

    krousdb NX-74205

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    I did find specs for the Toyo Versado Eco:
    Versado Eco | Toyo Tires

    The P195/65R15 has 1279 load, 51psi, 19 lb weight and 831 revs per mile, which matches the Ecopia revs per mile. I would think that the A29 would be similar.

    Also for comparison, the specs for the Dunlop SP 31 P195/65R15 has 1279 load, 44psi, and 836 revs per mile.
     
    #52 krousdb, Apr 22, 2016
    Last edited: Apr 22, 2016
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  13. cjecpa

    cjecpa Member

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    I guess it is better everybody's odometer is reading less mileage rather than higher. I think California allows odometer readings to be plus or minus 4% off. Would be more concern with manufacturers being on the plus side to speed up warranty expiration.
     
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  14. fuzzy1

    fuzzy1 Senior Member

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    The plus side has already been hammered away by class action lawsuits over warranty fraud. I know that both Honda and Subaru were hit up, as I received the legal notices for cars I owned (neither of which actually had the purported 'defect'). If other car makers were hit, I missed them due to not having subject cars.
     
  15. JohnF

    JohnF Active Member

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    Thanks for finding this. The difference in correction factors we see may be variation between different cars?
     
  16. cjecpa

    cjecpa Member

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    I think Nissan was the other. I am probably like most people and take it for granted that odometer is correct and more concern with the speedometer when switching out wheels and new tires.

    Now I am wondering if I am really getting the miles between each fill up or not.
     
    #56 cjecpa, Apr 22, 2016
    Last edited: Apr 22, 2016
  17. Mendel Leisk

    Mendel Leisk Senior Member

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    You will be getting slightly more miles, from what's been reported above.
     
  18. CR94

    CR94 Senior Member

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    That's exactly what most car manufacturers used to do, before the class action suits scared them into being honest, or erring appreciably in the opposite direction in an overabundance of caution. I checked odometers on a lot of cars in the old days. Most showed 1-6% high, a few even more. My own Subaru showed 5% high. My Mazda, 1.8% with the original tires. Cheating on the warranty was not their only motivation to exaggerate distance.
     
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  19. CR94

    CR94 Senior Member

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    Let's try again.

    Admittedly, "variables" might've been an imperfect choice of words on my part, so I'm sorry if that's the cause of your misunderstanding my point. However, note that I did not say every variable you can imagine is irrelevant. I wrote "those variables," specifically meaning reasons (some examples of which were described in the post I quoted) why the amount of fuel in the tank after fill-ups can vary. Obviously those inevitable filling inconsistencies can cause a sizeable short term error in the ratio of true mpg to displayed mpg.

    I hope we can agree that the average number of miles per gallon a car scores over a distance long enough to include several fill-ups should equal the overall distance traveled after an initial fill-up, divided by the sum of fuel volume added at the final fill-up plus all other fuel volume added in the meantime. That means filling inconsistencies in the intermediate fuel additions (or whether they even approximate complete fills) are---yes, I'll say it again!---totally irrelevant. Small inconsistencies between the initial and final fill-ups are not 100% irrelevant, but will diluted to the point of practical insignificance.

    If, over any group of several consecutive purchases, I compare actual fuel put in the tank to the the total volume the car claims it consumed (considering that claimed miles divided by claimed mpg=claimed gallons each time I read and reset the trip meter), the ratio of claimed volume to actual volume is remarkably consistent. (Actual mpg to displayed mpg ratio would therefore be the same, ignoring for now the much smaller odometer and pump errors.) That means this car's fudge factor is consistent.

    If you've kept good records and similarly analyze the numbers from your car, then do that again for another period, you'll likely also find a consistent fudge factor. It probably won't be the same as mine, even on another 3rd generation, due to manufacturing tolerances, etc. I'm getting a 5% error, plus or minus a tenth or two. Other credible or semi-credible PC members have reported different consistent discrepancies, from maybe 4% up to about 8%. I don't know yet what the corresponding range is for the 4th generation. The fact that all of the 3rd-generation Prii mpg displays, even the best, are exaggerating says to me that a degree of exaggeration is very likely intentionally programmed-in by Toyota. If you'd find that to be acceptable, good for you. I don't like it, but have to accept it, and can correct for it by applying my fudge factor.
     
    #59 CR94, Apr 23, 2016
    Last edited: Apr 23, 2016
  20. I'mJp

    I'mJp Senior Member

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    If you are averaging averages, you are probably getting the wrong answer.
    See en.wikipedia.org/wiki/Mean , for measurements of speeds, mpg's, you need to use the harmonic mean.
    An arithmetic mean will give you consistently lower numbers.
    Or just keep raw data, miles and gallons, add up the totals and divide.
     
    #60 I'mJp, Apr 23, 2016
    Last edited: Apr 23, 2016