2010 Sonata Speedo malfunction

This speedo on this auto diesel Sonata was locked on 35 kph at all speeds, even when car was stopped with ignition off it still indicated 35 kph.
Thinking it may just be mechanically jammed I gave it a "tap' to see if it dropped back. No such luck. Eventually disconnected car battery for 30 seconds or so and it still indicated 35 kph even when battery was off. As soon as I started car after reconnecting battery though it dropped back to zero. Any suggestions? Many thanks John
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