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    Speedo chip

    I've recently become aware that my speedo reads high, after checking against dyno rollers and GPS speeds. I'm pretty sure I know why: the chip in my cluster says 3.89 on it, and the rear gears in the car are 4.10 (positive on this-I counted teeth.) I assumed the car had its original cluster when I bought it, and therefore had the original chip. But now it seems that either the cluster or chip was replaced with one from a 3.89-geared car. I 've since replaced the cluster, too, which works fine.

    I'm looking for a 4.10 chip for the late (5 green light SI) cluster. (Any difference?)

    But there's one lingering question. Why the hell would the chip affect the speedo reading at all? The speed input comes from the ring gear sensor, which doesn't care if the gear ratio is 2.0 or 6.0:1, as long as the wheels turn the same RPM. If driveshaft RPM was used to compute speed, I could understand the disparity, but that would also require tachometer and current gear information, and even then only work for a 5-speed car. WTF?
    sigpic

    Mike

    '91 325i track car. Mostly...

    #2
    3.89 refers to the month/year (I believe), not the differential ratio. And you are correct that the differential ratio has no affect on the pulse rate from the speed sensor.

    All BMW speedometers read high by design as mandated by German law (or so i was told). The speedometer is within specification if it reads high by 2.4mph +10% of actual speed.
    Last edited by jlevie; 02-24-2009, 06:17 AM.
    The car makes it possible, but the driver makes it happen.
    Jim Levie, Huntsville, AL

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      #3
      its probably just out of calibration. if you happen to have a motometer cluster you can recalibrate it as they built in a trim pot. the vdo speedo has hardwired resistors. as it happens i adjusted my motometer speedo about a month ago. it had been reading 10 mph fast at 60. now its spot on as far as i can tell.

      for reference: http://r3vlimited.com/board/showthread.php?t=95733

      if that happens to be your problem, i could provide more information regarding the setup i used to recalibrate mine

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        #4
        I have VDO. Old one was MM. I can live with error, as long as I know what it is and why. Thanks, though.
        sigpic

        Mike

        '91 325i track car. Mostly...

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          #5
          hey at least it works. i lived with no speedo in my e28 the whole time i owned the car. damn i sure knew the rpm to speed ratios in each gear by the time i got rid of it though

          '89 Alpine S52 with goodies

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            #6
            How high does it read? Mine seems to read 4-5mph fast at any speed.

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