Calculating Pulses Per Mile

Discussion in 'High Tech Retrofits' started by Keaton, Sep 16, 2019.

  1. Keaton

    Keaton New Member

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    Hey Guys, I recently just installed a new Tremec five speed in my 1970. I was wondering if any of you know what the pulses per mile would be. I have 3.73 gears in the rear end and the tire diameter is roughly ~25 inches. I’ve been manually putting values into my NVU Gauges and 54,350 seemed to be the closest. Thanks!
     
    Last edited: Sep 16, 2019
  2. ULTM8Z

    ULTM8Z Veteran Member

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    If you can program it electronically, then basically whatever the pulse/mi number is that gets your speedo to read accurately is the right number.

    Generally speaking it’s a function of the axle ratio, the tire diameter, and the number of teeth on the VSS reluctor wheel. But I think the math works like this…

    For example, let’s say you have 26” tall tires, a 3.73 axle ratio and your VSS has say, 10 teeth on the reluctor wheel.

    For every revolution of the tire, you’re going (3.14)(26) = ~82” or ~7ft. 1 mile = 5280 feet, so you’d need ~754 revolutions of the tire to go 1 mile.

    For every revolution of the tire, you’re getting 3.73 revolutions of the driveshaft. So that would be ~2813 driveshaft revolutions, and therefore 2813 reluctor wheel revolutions.

    So then if you have 10 teeth on the reluctor wheel (every time a tooth goes by the magnetic pickup, that’s 1 pulse), you’d have 10 x2813 = 28,130 pulses per mile.

    For example, on my setup, I didn’t count the reluctor wheel teeth so I don’t really know what I have. But with the ECM reading the VSS in my Gear Vendors, I ended up programming in 115,000 pulses per mile to get the ECM seeing the right vehicle speed (I basically calibrated against a GPS Speedometer app on my smartphone).



    Like I said, I don’t think it really matters what number you end up with as long as the speedometer readout is accurate.
     
  3. Keaton

    Keaton New Member

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    Well the speedometer reads perfectly above 20 miles per hour, but any lower and the speedometer starts to bug out. I found that going higher than 55,000 makes the speed slowe
     
  4. Keaton

    Keaton New Member

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    I meant to say 54,350 works pretty well in my original post, not 35,350 pulses.
     
  5. ULTM8Z

    ULTM8Z Veteran Member

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    Could potentially be an electrical noise issue.

    What do you mean by bug out? Jumpy needle? What VSS are you running?
     
  6. Keaton

    Keaton New Member

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    Well the speedometer reads perfectly above 20 miles per hour, but any lower and the speedometer starts to bug out. I found that going higher than 55,000 makes the speed show slower and below 54,000 makes the speed read higher.
     
  7. Keaton

    Keaton New Member

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    The speedometer starts to jump around when you get to 10-15 MPH, but once you clear 20 MPH it reads just fine. I’m using the Tremec Electronic speed sensor on the rear tail shaft passenger side. (TKO500)
     
  8. ULTM8Z

    ULTM8Z Veteran Member

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    Does it do that no matter what gear you're in? If its noise, wondering if it's related to engine rpm...

    Could potentially reroute the VSS wires to a different location. Could be passing by something noisy that's coupling onto the VSS wires.

    Also is this a two wire or three wire sensor? Is there a ground with it?
     
    Last edited: Sep 16, 2019
  9. Keaton

    Keaton New Member

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    It’s a two wire sensor with no polarity. One goes to ground and the other goes directly to the speedometer.
     
  10. ULTM8Z

    ULTM8Z Veteran Member

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    Does it actually say to ground one of the wires? I thought typically these sensors generate an AC signal and that both wires would go to the device that's using them.

    What sensor are they running in this transmission?
     

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