Could it be some form of ratchet or clutch in either the speedo or gearbox as after all the speedo does not show speed when going in reverse, it may not be clearing or releasing as it should.
 
Could it be some form of ratchet or clutch in either the speedo or gearbox as after all the speedo does not show speed when going in reverse, it may not be clearing or releasing as it should.


Nothing like that in box or speedo, as Kermit says likely worn/damaged cable, the next symptom you may see is the needle jumping when going forward.
 

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