On a boring groomer day last year, I turned on my GPS, reset my track,
and did a speed run, just to see how fast I was going. My GPS (an old
Magellan Sport track) has a "maximum speed" indicator (I'm sure most
others do too), and when I got on the lift I saw that I maxed out at
72mph! I'm pretty sure the GPS speed indicator is accurate, as I
often compare it to my car speedometer, and it is always the same. I
was indeed hauling a$$, but was still totally in control, and felt I
could have pushed it a bit more (not too much though).

Not one to encourage dangerous behavour, but is something worth trying.