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Speedometer gearing

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  • Speedometer gearing

    I did a long journey in unknown territory so fired up my satnav. Horror of horrors I discovered that my Speedo is showing 60 at 65mph by the satnav. Usually they read about 5mph over. So I have been unknowingly speeding all this time.

    I can only suppose that it is because I'm on 15 inch wheels rather than the standard 13 inch for the Sierra donor car. Gearbox is an mt75.

    Does anyone know if you can get different gears for the Speedo drive?

  • #2
    Re: Speedometer gearing

    Tyre size can also affect the rolling radius. Different diff ratios from standard (for the gearbox) will also affect the Speedo. When I changed the diff ratio on my Hunter I fitted an electronic speedo with a sensor driven off the prop shaft. It’s pretty straightforward to then calibrate this if you follow the instructions. My speedo is now spot on. Another alternative is to fit one of the GPS ones. The snag with those is that they don’t work in tunnels! If you have a Ford gearbox it may be possible to change the driven gear which just pulls out of the side of the box. “Just” may take a bit longer if the cable outlet is obscured by body panels! If it’s necessary to change the drive gear on the gearbox mainshaft then that’s a major strip down job.


    • #3
      Re: Speedometer gearing

      There are companies that specialise in matching mechanical speedos to gearboxes. Some gearboxes, like the Ford type 9, have a range of speedo drive ratios available.


      • #4
        Re: Speedometer gearing

        I can tell everyone how to calculate speed from rpm and speedo calibration if you are interested. Most Lucas speedos have the number of turns per mile printed on the outside rim of the speedo dial. From memory (so this could be wrong...) spitfires had 1000 turns per mile speedos, as did triumph 2000 saloons, and 2500 saloons had speedo at 845? or something like that. Ford speedos are ALL calibrated to 1000 turns per mile as far as I know. Some specials (like Cossie) may have different.

        They key is being able to calucate turns per mile for your tyre sizes..... rolling diameter of radial tyres is based on -
        Tye Specs are WDT/ASR-WHS

        WDT = Tread width in mm
        ASR = Aspect Ratio ratio (number)
        WHS = Wheel size in inches

        Rolling Diameter = 3.14159 x [(WDT x ASR/1270) + WHS]
        which is = dia of wheel + two heights of tyre

        turns per mile = 63360/Rolling dia

        63360 = inches per mile, but 65261 works better (3% derate for tyre wall sag).

        then you can get prop shaft turns per mile, and then sort out the speedo gears.....or get speedo recalibrated


        • #5
          Re: Speedometer gearing

          I think what you really need is the number of turns per mile of the cable to the speedo. This is what Speedy Cables ask for to calibrate a speedo https://www.speedycables.com/speedometer-calibration/