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Big Twin
Posts: 1,956
Join Date: Mar 2005
Location: Normally in my own little world
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never done it on a 748, but I have taken a few car speedo's to pieces. Generally there will be a gear drive from the end of the cable drive and the tumblers that show the mileage.
Remove one of the grears ot tkae it out of mesh, then it's a simple case of rotating the tumblers to give the correct mileage. Replace the gears and re-build the speedo.
Alternative would be to buy a new speedo and keep the old one to show the mileage at the clock swap.