Might be more than you want to spend but I pulled this out of mine and went more stock/Dakota Digital setup
That sounds like a bad cable or speedometer gears to meThe car is original, I think the speedometer is not functioning properly. Between 0-10 it reads nothing, 10 - 25 it bounces all over. Above that it progressively less than the car is actually going. Odd part is the faster you go the farther off it is. Ocationally it stays between 55 and 60 when accelerating and then jumps up. I think something in it is loose or sticking.
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