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If ECM is pulling up the speed from the EBCM, since the speed sensors are connected to the EBCM, then problem is in the VSS wiring to the dash and Hud instead.
So on that note, would pull the ICP to check the VSS connector wire on the connector, since it sounds like the problem when an Hud was added to the car and spliced into the VSS wire/connector. If car came with Hud, then check the VSS wire up line to see if was cut or cracked isntead.