ODO vs. Run Distance since Last DTC Clear?

Discussion in 'Gen 2 Prius Technical Discussion' started by 2009Prius, Dec 22, 2010.

  1. 2009Prius

    2009Prius A Wimpy DIYer

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    Fellow PCer planetaire kindly provided the PID for the "run distance since last DTC clear" and I have been collecting data for a couple of days hoping to turn that into the odometer reading in my PriiDash(TM) project. When I plotted up the data against the car odometer reading I was surprised to find the slope was not one. :confused: If I force the slope to be one the fit is worse. The interception in either case does not make sense. There was never any DTC cleared (car bought new). See attached charts. Any idea what's going on?

    [​IMG][​IMG]
     

    Attached Files:

  2. 2009Prius

    2009Prius A Wimpy DIYer

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    Update: more data collected and the strange trend is still there:
    [​IMG]

    [​IMG]
     
  3. planetaire

    planetaire Plug in 20 kWh 85 km/h or > 208km range

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    Hello, 2009Prius

    I won't answer your question. But remember that "run distance since last DTC clear" is limited to 65535. On my car this pid don't change now.

    Why don't you use wheel rotation (for exemple pid 0B4, byte7&8), add all your travel, and store the result in a file for next trip ?

    :)
     
  4. 2009Prius

    2009Prius A Wimpy DIYer

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    Thank you! That's what I did in the end. It's just curious why the two does not agree.
     
  5. Patrick Wong

    Patrick Wong DIY Enthusiast

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    Have you noticed that, when you select the hidden diagnostic screens in the MFD and find the one that shows vehicle speed, that the speed displayed per the MFD is not the same as the speed displayed in the speedometer (even if you set the mph/km switch to km)? The speed per the MFD seems to be based upon periodic sampling vs. the speedometer reading which is updated more frequently.

    I am wondering whether this might be the reason for the minor error in run distance vs. odometer reading that you noticed?
     
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  6. 2009Prius

    2009Prius A Wimpy DIYer

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    No I haven't dared to drive around with the diagnostic screen, but I did notice that the various speeds reported by known passive CAN IDs don't always match the speedometer display. I think the speedometer display usually reads somewhat higher than the actual speed in order to comply with some government regulations. On the other hand one would think that the odometer reading should be accurate and consistent.
     
  7. Mike Dimmick

    Mike Dimmick Active Member

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    The displayed speed in the vehicle signal check is computed by multiplying wheel rotation speed by assumed circumference of the tire. The speedometer reading is fiddled.

    The algorithm over here - determined empirically - is that the speedo reads +10% below a displayed 55mph (real 50mph), +5mph over 50. This is to comply with the speedometer regulations which say that the speedometer's value must never be less than the real speed, and must not be more than +10% plus 4km/h at 40, 80 and 120 km/h. The tire's circumference can vary by about 2% in use, as it wears down, and there is always some variation between manufacturers and designs with the same nominal tire size.

    I believe that on the UK 3G Prius the fiddle factor is reduced to 5%.

    I can't find the regulations at the moment, but I'm pretty sure the odometer is not allowed anything like the leeway of the speedo.
     
  8. liskipper

    liskipper Member

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    I may have missed it, but how do you get to the hidden diagnostic screen?
    Thanks.
     
  9. Calle

    Calle New Member

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    Has anyone delved into this deeper? I have been battling this problem for many years. I first noticed this: When using GPS (several different devices) I notice that if my speedo says 67mph, my actual speed is 65mph. I have verified this with GPS and radar. Also, Priidash shows the actual correct speed while the speedometer shows higher. Anyone know if there is a way to solve this? My qualm is that if the speedo is off by 5% or so, then my odometer is essentially 5% off and I have 5% more mileage than I should on the car.
     
  10. 2009Prius

    2009Prius A Wimpy DIYer

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    I think the odometer should be very accurate. It's just the speedometer got inflated due to government regulations.
     
  11. fuzzy1

    fuzzy1 Senior Member

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    Why didn't you use the same GPS test to check your odometer too?

    As 2009Prius stated, the speedometer reads high due to government regulations.

    But it would be idiotic these days for any car manufacturer to bias the odometer measurably high, especially after the class action warranty fraud lawsuits that struck several makers -- I received notifications for Honda and Subaru, both of which made settlements including a 2% warranty extension. But my own cars were reading much closer to correct than that.

    While my Prius speedometer read high similar to yours, its odometer read about 0.2% low when the tires were near 20k miles.
     
  12. Feri

    Feri Active Member

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    I've noticed that when I reset either trip A or trip B while driving at a constant speed there is a close correlation between the average speed reported after a fairly short time, and the gps reading. (??)

    I've also noticed my Ultraguage speed reading is 1-2 Kph lower than the gps.
     
  13. Calle

    Calle New Member

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    I have checked the ODO vs the speedOmeter. If my speedo says 70mph, I am actually doing 67mph. I compared the ODO reading after a 300 mile stretch and sure enough it was off. In other words, my ODO would say that I've driven 70,000 miles but in reality I've only gone 67,000 miles. This is a huge difference.
     
  14. uart

    uart Senior Member

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    What did you compare your ODO with? (to know that it was off by 67/70)
     
  15. fuzzy1

    fuzzy1 Senior Member

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    As Uart already asked: compared the odometer to what? A speedo doesn't work directly because it doesn't measure the same thing.

    I have compared my odo to several different measures:
    (1) multiple 5 mile roadside speedo/odo check stations;
    (2) 100+ miles sections of interstate highway with mostly consistent mile markers (see previous discussions for some of the snags);
    (3) automotive GPS;
    (4) hiking GPS in continuous reception mode (good);
    (5) same hiking GPS in battery-saver mode (serious errors).

    The battery saver mode produced errors in the 5 to 20% range, depending on terrain.
     
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