i know that the guage cluster in the Chevy is a little different than my GMC, but I still believe that it has a basic mechanical speedometer. It is the simplest fix for the entire guage cluster. The cable either came loose from the back of the cluster or it came loose/broke near the mounting point on your transmission. I had a similar problem on my truck. Turns out that the speedo cable caught on something and ripped out of the transmission. I bought a new cable and had it installed in under 20 minutes...problem solved.
