I have a 2004 Envoy which just recently experienced this same issue. Speedometer at 120 mph +. Where do I find most recent info. on this from GM? How do I find the ground location mentioned? Dealer here in Florida says replace the entire instrument cluster. Everything else seems to work, other than vehicle is no longer level and leans to right. Seems to be related to automatic air shocks??? HAd check engine light but that goes off.