Speedometer readings would seldom match exactly with GPS as tire wear and inflation pressure etc would affect the actual reading and a variation of about 3 % is expected.
As to rpm not registering correctly and the speedometer suddenly going out of specs, it can mean a fault with the electrical circuit of the meter cluster. A connection could be loose or has poor continuity, high resistance in the ground circuit.
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Thursday, February 14th, 2013 AT 7:13 AM