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On mine, the cruise overreads too, though not by as much as the speedo. By way of an example, 50mph is about 52 on the cruise and around 54 on the needle. That's nearly a 10% overread!
These figures are verified by GPS using a TomTom GO 700. GPS is accurate to within 1mph on straight and level ground, though bends and inclines do tend to throw it (when going up or downhill, GPS will read lower than your actual road speed.) For most purposes, you can take a good GPS reading as gospel.
Bear in mind that a speedo can never be 100% accurate, as the rolling radius of the wheels and tyres is affected by wear and inflation.
Currently in Horatio's garage:
2010 Laguna III Initiale.