Thinkng.
I'd been mulling this very question over for a time as I didn't fully understand the whole overspeed at high alt thing.
Am I correct then in saying that the red overspeed dial is basically marking the point where undesirable aerodynamic effects begin? I understand why this limit comes down as you climb but now I see its because the air is thinner (doh!). I used to look at it and think, I can fly at 400kts sub 10000 so why am I limited to 320kts at 35000? Obviously I am "going" much faster with respects to ground speed but used to think the overspeed limit was to do with aircraft mass and ground speed (ie structural limit) not air density related.
You learn something new every day eh?
BP