When you're just sitting does it read 20 mph? The instrument cluster analog or digital? It could just be that you need to reset the zero on the needle if it's analog and that is the case, otherwise I don't know. If it's digital maybe there's a short adding unwanted voltage somehow? It doesn't make sense that it would always be 20 mph off. I think the speedometer cable hooks directly into the transmission so thats where you would look I'd guess.
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