This happened on my 1969 MG Midget. It was a broken speedo cable. I realize this is no help to you.
Your issue may be due to a broken or disconnected wire or sending unit.
Read detailed trip reports, see photos and videos on my travel blog, luinil.com.
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Current van: 2002 Ford E350 extended body camper with Colorado Camper Van pop top and Agile Offroad 4WD conversion.