[brianlay]

If the odometer is correct that tells us the pickup/wiring/magnets on the ring gear are all ok. I know NH said the speedo is ok but it sure seems like that's where the issue is.
Does the car have cruise control? If so unplug the connector to the control unit under the dash. The pick also feeds that unit and maybe it has a problem...although i've never heard of that. Easy to try though.
Otherwise it points to the speedo. Is the needle possibly sticking?