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Thread: How to scan speeds faster than 158 mi/h (255km/h)?

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  1. #1
    Member Treurentner's Avatar
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    Default How to scan speeds faster than 158 mi/h (255km/h)?

    Hi,

    if I use the SAE VSS pid it works fine until the limites are reached.
    If I try to rescale it in EFIlive it has the new max level but still stops at the limit, What causes this? Is it an SAE hardware limit? Can it be changed?
    I can calculate the "real Speed" by using a correction factor to the RPM pid in last gear but this should be only a band aid.
    At the Autobahn here in Germany I tried to do a topspeed run with our supercharged Corvette C5 and was hoping to see 200+ mi/h (320+ km/h) and so it would be really useful !
    Anyone else already noticed this problem?

    many thanks & best regards from Germany

    Marco

  2. #2
    Joe (Moderator) joecar's Avatar
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    Right click on SAE.VSS and go More Info...

    Quote Originally Posted by ScanTool->PIDs->SAE.VSS->More Info
    Vehicle Speed Sensor {SAE.VSS}

    Units: kmh
    Range: 0..255 kmh

    Units: mph
    Range: 0..160 mph

    VSS displays the vehicle's road speed if utilized by the control module strategy. Vehicle speed may be derived from a vehicle speed sensor, calculated by the PCM using other speed sensors, or obtained from the vehicle's serial data communications bus.

    Reprinted with permission from SAE HS 3000/2003 © 2003 SAE International.
    This pid is 1 byte in length, so it appears that 255 (all 8 bits of the unsigned byte are 1) is the maximum value in km/h (and this converts to 160 MPH).

  3. #3
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    Quote Originally Posted by Treurentner
    Hi,

    if I use the SAE VSS pid it works fine until the limites are reached.
    If I try to rescale it in EFIlive it has the new max level but still stops at the limit, What causes this? Is it an SAE hardware limit? Can it be changed?
    I can calculate the "real Speed" by using a correction factor to the RPM pid in last gear but this should be only a band aid.
    At the Autobahn here in Germany I tried to do a topspeed run with our supercharged Corvette C5 and was hoping to see 200+ mi/h (320+ km/h) and so it would be really useful !
    Anyone else already noticed this problem?

    many thanks & best regards from Germany

    Marco
    Yep got the same thing here, Marco.

    Joecar , Thanks for the heads up .
    Roy 98SSSSSS

  4. #4
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    Can you get by the max limit by creating a calculated PID using the transmission output shaft speed. I have done this to verify the speedometer accuracy and improve speed resolution.

  5. #5
    Joe (Moderator) joecar's Avatar
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    Quote Originally Posted by blandmiller
    Can you get by the max limit by creating a calculated PID using the transmission output shaft speed. I have done this to verify the speedometer accuracy and improve speed resolution.
    Yes, that should work, create a calculate pid to give you road speed from TOS, which you already did.


    The calc pid would calculate this:

    road_speed[mph] = (tos[rpm] * tire_diameter[inch]) / (gear_ratio * 336)

    .
    Last edited by joecar; September 7th, 2006 at 04:25 AM.

  6. #6
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    Quote Originally Posted by joecar View Post
    Yes, that should work, create a calculate pid to give you road speed from TOS, which you already did.


    The calc pid would calculate this:

    road_speed[mph] = (tos[rpm] * tire_diameter[inch]) / (gear_ratio * 336)

    .
    What is the "336"? Is this the gear ratio or a multiplier used on the "gear_ratio"? (so if my rear end gear ratio is 3.25, would I use 325?)
    Or, if I'm interpreting correctly from previous posts, the "gear_ratio" is the currently selected gear's ratio, and the 336 is the differential gear ratio. Does a separate PID need to be created for "gear_ratio"?

    Thanks

  7. #7
    Joe (Moderator) joecar's Avatar
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    Quote Originally Posted by Blue70SS View Post
    What is the "336"? Is this the gear ratio or a multiplier used on the "gear_ratio"? (so if my rear end gear ratio is 3.25, would I use 325?)
    Or, if I'm interpreting correctly from previous posts, the "gear_ratio" is the currently selected gear's ratio, and the 336 is the differential gear ratio. Does a separate PID need to be created for "gear_ratio"?

    Thanks
    Hi Blue70SS,

    The 336 is simply the constant of conversion from mph to rpm and inches (and/or vice-versa); it is not related to rear axle ratio.

    so for example (using say 6th gear):

    TR6060 6th = 0.63
    your rear axle = 3.25

    using engine RPM:
    gear_ratio = 3.25 * 0.63 = 2.0475
    road_speed[mph] = (engine_speed[rpm] * tire_diameter[inch]) / (2.0475 * 336)

    using have transmission output rpm:
    road_speed[mph] = (trans_output_speed[rpm] * tire_diameter[inch]) / (3.25 * 336)


    I don't know if there's a pid for the M6 trans output speed in RPM.
    Last edited by joecar; February 22nd, 2018 at 12:06 PM.

  8. #8
    Lifetime Member wesam's Avatar
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    Sorry to pull back old thread
    Joe What tos speed stands for ?
    for the gear ratio do you mean the rear end ratio or the selected gear ratio ?

    Thanks
    EFILive Tuner

  9. #9
    Lifetime Member Mitco39's Avatar
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    Trans output shaft.

  10. #10
    Joe (Moderator) joecar's Avatar
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    TOS = trans output speed

    gear ratio = trans current gear ratio x rear axle ratio

    edit: correction: if you're using TOS then gear_ratio is simply just the rear axle ratio.
    Last edited by joecar; February 22nd, 2018 at 12:00 PM.

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