Speedometer correction after gear swap

Discussion in 'Transmission & Drivetrain' started by bhaunold, Oct 22, 2019.

  1. bhaunold

    bhaunold New Member

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    Hello, I’m getting ready to replace my rear end, I’m wanting to put a performance pack rear end in the car with the 3.55 gears. My cars currently a base model automatic with 3.15 gears. I was wondering if I will need to get a tune for 3.55 to have the Speedo be accurate again. Or since the wheel speed sensor is on the hub of the gears will affect the speedometer at all? Also will it affect shift points at all with stock tuning or if it’ll just rev quicker. Thanks in advance.
     
  2. OP
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    bhaunold

    bhaunold New Member

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    When you say tiny are you talking about 1-3 mph? I’m okay with it being 5 off because I drive 5 over everywhere.
     
  3. mavisky

    mavisky Well-Known Member

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    S550 chassis cars measure vehicle speed through the wheel sensors. You can change gearing and the speedometer will remain accurate. Only changing the outer diameter of the rear tires will throw off the speedometer on these cars. This has been proven multiple times, but the incorrect information persists because this is the first chassis in which this is the way the speedo operates.
     
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    bhaunold

    bhaunold New Member

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    Thank you for confirming this. I’ve been trying to figure out what is true and false all day. Would it affect shift points at all?
     
  5. Freedom

    Freedom Well-Known Member

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    Whipple and ford have tools that adjust gear ratio. I think it’s called flare tool for whipple and the ford one comes with the performance pack upgrades.
    I’m not sure what happens if you don’t adjust the ratio with these devices though.
     
  6. mavisky

    mavisky Well-Known Member

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    Perhaps slightly. Determining the ideal shift point to the most finite measurement is predicated on the amount of power delivered to the wheels at any given RPM. The gearing change can have an effect on this.
     
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