Speedo calibration

I recently had the coil replaced on my zetec 1596cc estate due to misfiring and when I drove the car away from the dealership the speedo was only registering a fraction of the true road speed. I took it straight back
and they said (it was a Ford dealer) that the speed sensor had gone and needed replacing and it was a coincidence that had it happened while they were fitting the new coil.
They broke the old speed sensor and said they couldn't remove the rest without taking the gearbox off, so I had the new sensor fitted at another garage (without gearbox removal). However, the speedo on the dash was behaving exactly the same (registering only a fraction of the true road speed) as when the old speed sensor was in.
I took the car back to Ford who recalibrated the speedo and now it is fine. They said the result of their diagnostic test was different when they said the speed sensor needed replacing to when the new speed sensor had been fitted and they said it needed recalibrating only.
Yeah right? A broken speed sensor gives identical read out to dashboard as one that only needs recalibrating? Didn't it just need recalibrating and not replacing of the sensor in the first place? I've contacted their head office who are investigating: they say reprogramming following replacing the coil wouldn't have affected speedo calibration and they are now waiting for the diagnostic code records from the dealer.
Does anyone know what causes the speedo to lose its calibration and is the dealer's argument that the speed sensor was faulty plausible, given that when the new one was fitted it was behaving in exactly the same way? Isn't it much more likely that they got their diagnosis wrong in the first place? Or would a failing speed sensor have put the calibration out perhaps?
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misfiring
Thius is a strange one, and the only explanation I can think of is a result of the failing coil pack. When they go they often spike the ECU with high voltage. The ECU is clever enough to shut itself down to prevent damage in these cases. The instrument speedo is driven directly by the ecu, which may have "forgot" its calibration figure for the speedo during the period the coil pack was faulty.
Any one else have any ideas?!
Tim..
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Thanks, Tim. The only thing is the speedo was registering fine all the time the coil pack was playing up. It only came up with wrong readings after the new coil pack had been fitted.
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