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.