The car is a 64 Valiant, and all was well when the speedometer suddenly shot up past 100mph with the car doing about 30mph. And a loud, spinning/grinding sound started coming from behind the dash.

Since I was close to home, I headed back while experimenting a bit. When driving under 20mph, there is no grinding noise and the speedo reading is reasonable, if not correct. Upon going just a little faster, the grinding starts and the speedo jumps way up, sometimes going all the way around past zero.

So it seems that there is a problem with the gauge, the cable, or the transmission...or some combination of the three.

Does anyone have a suggestion as to a good way to figure out what is causing the problem?

Am I missing a possible culprit?

Bearing is shot in your speedometer, letting the driven cup sag down to the point where it physically hits the stirrup, which flings it hard against the stop.

Pull the speedo head and send it to Deluxe Speedo, see here.

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