speedo repair and recalibration

Anybody know how to recalibrate a late model Smiths speedo? I think the speedo cable I was using was too long and was forcing the magnet too close to the cup resulting in the speedo reading way too high. When I opened up the speedo, there seemed to be a lot of slop in the internals allowing the magnet to move up and down about 1-2 mm. This means that when you put some force on the cable, the magnet gets closer to the cup and the speed goes up. Anyone seen this before, and if so, do you have any recommendations as to how to repair? Thanks.

A professional repair is really the only option if the unit has worn like that from having a too long inner cable fitted.

L.A.B. said:

A professional repair is really the only option if the unit has worn like that from having a too long inner cable fitted.

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Thanks L.A.B. I was afraid you might say that!!