Speedo turns per mile.

[twolitre]

Here I go again.  My Triumph 5T has several non-standard features (to say the least).  It came with a magnetic speedo instead of a chronometric.  I see no reason why I cannot use a magnetic for the time being, but not yet being able to run it and with the speedo cable also missing I cannot try it.
  The numbers on the Smiths magnetic indicate 1000 turns per mile.  Can anyone tell me if this is suitable?  Or if not can anyone tell me the turns per mile needed.
  I know it can be worked out from pushing the bike a carefully measured distance and counting the drive turns then doing a little simple maths, but the bike is not yet in any state to do that and if I need to track down a different speedo I don't want to wait until the bike is complete and lacking a speedo.
Jim.

[L.A.B.]

A large number of the later Smiths magnetic (MPH) speedos are 1000 turns per mile units, but of course these are meant to be driven from a wheel mounted drive gearbox, the particular ratio of the drive box being matched to the wheel size, but I would think your speedo is driven from the gearbox which does complicate things slightly?

      

[twolitre]

Thanks LAB.  I'm not even sure that this is even intended as a bike speedo.  I suspect the chronometric was purloined somewhere along the line in view of the price they fetch even when broken.  I have been unsuccessfully keeping an eye open for the correct chronometric, but have not found one yet.
  Curiously, my Triumph Gentry (kit car) has a speedo with internals from a different (1000 turns) unit to correct it, so I have, at least, a unit I can cannibalise if my car speedo packs up.
Jim.

[L.A.B.]

You may know this already, the correct Chronometric MPH unit for your 1957 model is likely to be a Smiths S467/99/L  

[twolitre]

Thanks LAB. I did know, but I have not been able to find one which did not require an overdraft.  The only one I found was £270!!!
  It seems that this number only fits 5T's.  I expected the 6T at least to be the same, but it seems not.
Jim.