I've gone to ski trip at around 7,000 ft, once before Christmas and 2nd time after Christmas. During
2nd trip, my '03 Trailblazer speedometer went beserk, like when I knew I wasn't over 30 mph, it was reading 65 or something. When stopped, speedometer would be stuck at 55 or close, then 65 or so, etc. During these trips, I switched between 2WD and 4WD few times, and have used lower gear (2 or 3 instead of D) while cruising long downhills.A couple of days later, speed pointer would backtrack 5 mph or so every time I turn off engine, and eventually now it's back to zero when engine off. It's like nothing ever happened.
It can't be high altitude. I would like to know what may have been problem. Whatever was the problem, it can happen again, I would think. Any pointers? TIA.