the Speedo in my Max just stopped working a little while ago, it still trys and struggles to get up to the speed, but drops back down to 0 if i hit a bump or something. it's a 86 analog dash.
Your speedo cable might be coming loose. That could be why when you hit a bump, the speedometer goes to zero. I'm sure there could be other possible reasons why it's doing that, but that would be the first that I would check.
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