Yeah, 55 to 70 sounds about right - depending on conditions. Deep snow or cutting a new trail will be slower; hard packed will be faster. Heck, I haven't had my 550 liquid past 90 - but
I've never run it on ice or a really hard packed smooth trail.
That's also when using my GPS for speed tracking! The speedos on every sled I have ridden all read 10 or more MPH faster than you are actually going. That may be where the "90" numbers come from. Lots of people think their sleds will easily do 100+, but take 'em to a radar run, and they'll be telling a different story.