Speedometer is driving me crazy- Something is Wrong

Dana67Dart said
You have a GPS in your phone Google maps displayed the speed.

Drive 60 mph on the phone, look at your speedo.

There is a possibility that the cable is rounded at the ends and is slipping.

Or the speedo is internally messed up.
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Great idea, this could rule out the Cable if it is bad or not. But on the other hand, if the end was rounded and slipping then the speed on the dash would be slower and not faster than the Radar posted speed wouldn’t it?