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?