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Effective PSD gear ratio?

Discussion in 'Gen 3 Prius Technical Discussion' started by 70AARCUDA, May 4, 2014.

  1. 70AARCUDA

    70AARCUDA Active Member

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    Does anybody know what the 'effective PSD gear ratio' is for the GenIII Prius?

    I ask this question because the Toyota document "Application for Certification" to EPA lists a test 'N/V'-ratio of 26.8:1 ( N = engine rpm; V = vehicle velocity, which is 50 MPH ), and knowing that the GenIII PSD has a 'fixed' final drive ratio of 3.267:1, I was curious to determine the equivalent GEAR-RATIO the EPA-testing was performed at. So, I used the following useful equation and backsolved for G:

    MPH = [ 60 / (G*A)]*[ RPM / rpm ]

    where:
    MPH = vehicle speed, miles-per-hour (EPA test speed = 50 MPH)
    RPM = engine speed, revolutions-per-minute (1,340 RPM)
    rpm = tire speed, revolutions-per-mile (Ecopia EP-20: 195/65-15 = 833 rpm)
    60 = conversion coefficient, minutes-per-hour
    A = axle (final) drive-ratio (GenIII PSD = 3.267:1)
    G = gear ratio ( unknown )

    ...backsolving, I got G = 0.59:1, roughly. Does this sound about right?
     
    #1 70AARCUDA, May 4, 2014
  2. fuzzy1

    fuzzy1 Senior Member

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    In actual practice, the 'effective' ratio is quite variable.
    Under some conditions, this is about right. At 60 MPH, I commonly see 1300-1600 RPM in benign conditions, climbing above 2000 in less favorable conditions or moderate uphills. But depending on conditions, RPM can be anywhere from 992 (downhill) to 5200 (steep uphill).
    To check for yourself, use an engine monitor (e.g. ScanGauge-II, Torq, or numerous similar products) to check your own RPM.
     
    #2 fuzzy1, May 4, 2014
  3. TomB985

    TomB985 Member

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    I believe the whole point of a CVT is that the effective gear ratio is widely variable. I don't think you can put a number to it, because it depends on the speed of MG1.

    Toyota Prius - Power Split Device
     
    #3 TomB985, May 5, 2014
  4. 70AARCUDA

    70AARCUDA Active Member

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    The point of my question was to determine at (roughly) what overall effective gearing Toyota was claiming to EPA they were performing the EPA fuel economy & emissions tests.
     
    #4 70AARCUDA, May 5, 2014
  5. CaliforniaBear

    CaliforniaBear Clearwater Blue Metallic

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    Mileage is miles/gallons based on specified driving conditions. Why do you care about the gear ratio?
     
    #5 CaliforniaBear, May 6, 2014
  6. 70AARCUDA

    70AARCUDA Active Member

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    ...because: "...Enquiring minds want to know; the rest of us are just nosey..." (wink,wink)
     
    #6 70AARCUDA, May 6, 2014
  7. JimboPalmer

    JimboPalmer Tsar of all the Rushers

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    a final-drive ratio changed from the standard Prius's 3.27:1 to 3.70:1 for the Prius v

    I hope that helps somehow, as I do not fathom how to get a drive ratio for a CVT
     
    #7 JimboPalmer, May 6, 2014
  8. David Beale

    David Beale Senior Member

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    Well, there are two gear reduction "trains" in the Prius. The diff, which you already know about, and one other gearset.
    The ratio of that is posted on the web and is in the service manual (and I'm too lazy to hunt it down). It used to be a chain system in the GII, but was changed in the GIII to gears.

    And as already posted, it can't be "measured" by turning the engine and watching the wheels. ;) Due to the e-CVT.

    I would suspect Toyota listed the combination of the above two, which would be the effective gear ratio at "lockup" (which doesn't mean anything to a Prius HSD but I'm sure happens).
     
    #8 David Beale, May 7, 2014
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