My speedometer started messing up today. It sat at zero on the way home, yet when I got on the highway, it flicked all the way up to 120 to 130 MPH. I think I need to replace my speedometer cable and my speed sensor.
Your assumption sounds about right, either the sensor and or the cable are sending some mixed signals to the dash and it doesn’t know what to do with it. Have a certified mechanic take a look at your speedometer components ensure that this is in fact the problem and make the necessary replacements of the failed parts if necessary.