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Thread: Programming for Speedometer Correction?

  1. #1
    Tuner
    Join Date
    Sep 2007
    Location
    Hillsborough NJ
    Posts
    82

    Programming for Speedometer Correction?

    I am not sure how the speedometer calibration is done. I did an engine swap and installed a 3800SC/4T65eHD in a Pontiac Fiero. The speedometer seems to be reading high by about 10%. How do I make corrections?
    Now I see boxes for tire size, differential ratio and sprocket ratio but how do these relate? I am assuming that tire size means tire O.D. and that differential ratio is your FDR but what the other parameter boxes about (rev per mile, sprocket ratio) and how are these calculzted and set?

  2. #2
    Tuner
    Join Date
    Sep 2007
    Location
    Hillsborough NJ
    Posts
    82
    I cannot believe that no one including those at HPT knows the answer to this question. Surely if you change the sprocket or tire size its going to have an effect but which way do you skew the numbers?

  3. #3
    Potential Tuner
    Join Date
    Mar 2017
    Posts
    2
    Input smaller tire diameter in gear/tire tab,speedo reading faster speed usually larger tire, vss pulses,divided by vss per mile,= trans revs,remember go by your calculated tire size,my 235 55 17 is a 24.99 calculated diameter as physically 27.5 took me almost a year t figureni had to input 3.69 gears w 1,121 sprocket to get my 05 sc ss monte t shift rt