renton82 When you say "driving your car" I worry about that. Is the watch set to auto mode? I can see the watch getting confused while driving. Try locking in Altimeter mode and see what happens.
renton82 How fast is "fast?" A recent trail run with 5x seemed to do fine. I'll see what the 5x+ does this week as I'll be descending pretty quickly from 14,000 feet