I have seen 65 once, descending the east side of Monitor Pass at 8000' on an 8% grade. It is normal to do low to mid 50s in the high Sierras with the thin air.
Most fun downhill? Lake Tahoe to Carson City on HWY-50. Caught and passed a couple Harleys on a corner. Rest of the descent, they would pass us on the straights, we would pass them on the corners.
I have seen a person lose their life on a downhill, and watched another rider taco a carbon wheel at 45+. Listen to that little voice when it tells you it is time to slow down.