Speedometer readings

I recently had a new speed sensor fitted and Ford had to do a recalibration before the readings on the speedometer were right. Can anyone tell me other
circumstances that can lead to the speedometer readings losing their calibration? Would disconnecting the battery do this for example?
The reason I ask is because the speedometer was behaving in exactly the same way (i.e. as though it needed recalibration - although I did not make this connection at the time) before the new speed sensor was fitted: making me think with hindsight that the problem wasn't with the speed sensor at all (- although Ford reckoned their diagnostic equpipment proved this - but maybe that only picks up the output signal to the speedo and not the signal from the speed sensor itself?)
Thanks to Tim for one idea on this already in reply to my last post on this matter.

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