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Is the speedometer separate from the odometer?

Discussion in 'Gen 2 Prius Technical Discussion' started by F8L, Aug 21, 2011.

  1. F8L

    F8L Protecting Habitat & AG Lands

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    I ask because we all know the speedometer reads high from the factory. Is the speedometer system connected to the odometer system?

    My main point for asking is because I want to better understand how tire size will affect the MFD mpg calculation vs. hand calculations using the odometer reading. In my experience the MFD and the odometer miles are pretty consistent. This would lead me to believe that any tire change that effects the speedometer would also affect the odometer. Thus a tire that is larger in rolling diameter will give a more accurate mpg reading than the oem size which causes the speedo to run fast.
     
    #1 F8L, Aug 21, 2011
  2. xs650

    xs650 Senior Member

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    I haven't seen a car where they don't use the same signal source.

    The car electronics typically deliberately exaggerate the speedometer reading you see but usually make a good attempt at being honest about the miles traveled. If you do something like change tires to make the speedometer read x% different, the odometer will show the same percentage change.

    A larger diameter tire would make an optimistic MFD mileage readout more realistic, but the difference between the MFD and manual odometer calculated MPG would still be the same.
     
    #2 xs650, Aug 21, 2011
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  3. F8L

    F8L Protecting Habitat & AG Lands

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    That is how I assumed it worked but I wanted to be sure.

    When I measured the speedo with my oem 185/65/15s the speedo was 2mph too fast compared to numerous GPS readings. The new 215/45/17s made the speedo read closer to the GPS reading. So now I travel faster and the tires turn less revolutions per mile. So in essence I am reading more accurate mpg. Does this sound right?
     
    #3 F8L, Aug 21, 2011
  4. Corwyn

    Corwyn Energy Curmudgeon

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    Even if they used separate signal sources, they are still using the same data, i.e. tire revolutions. All changes to tire circumference, will affect that data.

    Until a car maker starts using GPS data for the odometer, or enables calibration of one or both (unlikely due to regulations).
     
    #4 Corwyn, Aug 21, 2011
  5. xs650

    xs650 Senior Member

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    It's just a matter of someone with the knowledge wanting to do it bad enough. They just have to decipher how the speedometer/odometer system works and change some numbers in the cars computer. Not an easy job, but far more complex things have been modified in automotive computer systems.
     
    #5 xs650, Aug 21, 2011
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