what is wrong with my speedometer?

The speedometer operates from data provided via a CAN network (data network) from the vehicle speed sensor and engine's processor. -The sensor could be faulty -The gear driving the sensor could have issues -The network could have a problem -The wiring could be a problem -The speedometer itself could have an issue. Testing is required with a proper diagnostic tool to find out what's going on.

http://www.gmtruckclub.com/forum/threads/2005-chevrolet-silverado-speedo-acting-crazy.78317/

These 03-06 Silverados are bad about this kind of problem, it's caused by a failing speedometer stepper motor inside the instrument cluster. Each gauge has a motor that controls it's needle movement and when the motor goes out the gauge will either stick, jump around or die all together. If you'd like to learn more about the problem and how to get it fixed please visit our site at www.drspeedometer.com Thanks, Fred Dr.Speedometer