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correct chronometric speedometer

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Hello All,
I'm hoping someone can help me find the correct speedometer for my 1950 Model 18. The one I have is a 120 mph unit, but is fairly accurate, at 1475 revs/mile. I also need a bottom-pull trip reset, as mine is a side pull. 

I would be grateful for any information as to purchasing or rebuilding the correct unit. I lived with it the way it is for 40 years, so now might be a good time to put it right. Thanks for your help.
Jamescurrent speedo

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Chronometric instrument services (chronometrics.co.uk) has a big stock, and there are several others.  Your 120mph instrument might well be more valuable than the 80mph version.

I set up a spreadsheet when I was looking for a speedo and by my calculation, if it is geared to 1475 turns per mile, your instrument will read 5% fast if you have a 21" rim and 3" section tyres.  I think the only Norton matching that will be prewar Inters (model 30 and 40) or post war Manx and 500T.  Yours comes out 14% fast.

 

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