HXsman, most if not all new style ignition systems work on a square wave signal (digital) for controlling coils, including ICE. All Holden electric tachos including sandman pre E.F.I engines all use a different wave signal for the input driver. Some automotive Instrument repair company's can convert your original tacho to except the different input. The new design distributors with square wave/digital triggering system are far superior system than the old points set up. So this is probably causing your tacho to read incorrect.
However over the years I have had a few Holden tacho needles warp due to heat and touch the plastic lens cover and stop at that contact point.
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