The problem appears to be the stepper motors in the instrument cluster that drive all of the gauges. They are junk, and often go bad. There is a new and improved one that fixes the problem. DO NOT give GM the satisfaction of charging you hundreds to fix this. If you are ok with a soldering iron, you can fix the speedo with a new stepper motor for a little more than $12.00. There is a guy on eBay who sells the stepper motors for less than $10, and a couple of bucks for shipping. They are a newer generation of motors, and supposely don't go bad. He gives you instructions on how to do the repair with only a soldering iron, and a solder wick. I ordered 7 motors to replace them all for less than $75. The good motors have the following part #: X 25 168. The bad ones are: X C5 168. If you're not comfortable with the soldering, this guy will do it for about $150, but you need to ship the whole instrument cluster to him. It's very easy to remove, but you'll need to run without the cluster while it's out for repair. There are a number of people on the web who will repair or replace. Some ship you a repaired one, and require you to send your "core" back. Bottom line, there are lot's of options. Don't give GM the satisfaction or your $. You can find one or two sources of these motors on e-bay. Search for 03-06 GM Cluster Chevy Tahoe Silverado Stepper Motors. You can find lots of people who repair or replace the clusters. Good luck!