I would start by checking the speedo cable and gear in the transmission.
I recently changed the speedo gear in my M20 to correct for a new ring/pinion set and saw something similar, where the speedo needle would stay at a certain level and not move. The odometer worked fine. The new speedo gear came in a two pack, so I installed the other one and it works fine.
What I found is the speedo cable end didn't seat fully into the trans gear. I'm thinking it forced the cable end to push in too far at the speedometer side of the cable.
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1970 GTO-vert, 461, eheads, stump puller, m20, 3.42
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