Chevrolet Repair: 2004 silverado, speedometer needle, driven wheel

Question
CJS,
 I own a 2004 silverado.  I was coming to work today and the speedometer needle gradually went moved all the way past 100 mph to where it is facing straight down.  I tried stopping and restarting and it did not reset.  Do you what happened?  If so, what needs to be done to fixed it?  

Answer
Hi Robert,

Does it stay pegged at 100, when the car is stopped?  Did you turn on any electrical devices just before it happened?  Did you install any electrical systems recently?

Speedometers work by way of the cable turning a wheel behind the needle.  Through magnetism, the cable driven wheel turns another wheel connected to the needle.  This magnetism, can be interrupted by stray electrical magnetism waves interrupting the speedometer magnetic system.  I would look for something that may be sending too high of a current, and is located near the speedometer.  Otherwise, the speedometer may have just went bad (out of sync).  If so, you can replace it with another from a junk yard.  Watch the mileage on this one, for resale issues.  

Hope this helps.

C J S