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Re: Magmo speedometer revs/mile

Hopefully by now Bob and Sandy will have gone through Sandy's pile of Magmo bits and can help me identify a couple of Magmo speedos that I have recently pulled apart! These are most likely to have come from a Box Saloon or a Ruby but although the dial on one is marked X23520, the markings such as 1040 or 1080, to indicate the turns per mile (TPM), are not there. Perusing threads on this site seems to suggest that the former goes with 3.50 x19 tyres and the latter with 4.00 x 17. These ratios are very much the same as the respective tyre constants of 775 and 800 quoted in the Bill Williams' book on Specials. In any case the % difference is not that great even if they are wrongly matched.
I understand there are other Magmos around with TPMs that are seriously different. So, my question is: what is the identifiable difference between the components when you have them stripped down? The worm drive on the ones I have has a step-down ratio of 20:1, using a single start thread. Perhaps this stayed constant and the teeth on the ratchet is the variable. Then there's the speedo; is there a variation in the magnet or is it in the hair spring? Finally there's the possibility that someone in the past has swapped the dial face for the wrong one!!
It's tempting to mix and match to get a decent looking unit but it would be nice to plan for a good outcome rather than "an iterative series of road tests"!!! Hope someone can add some info' on this topic
Ron

Location: Thornbury