"Hell or Melbourne in 60 days" on the route they followed (down to the Roaring Forties, then east, would mean at least 250 miles per day every day, which would mean 10.4 miles per hour or about 10 knots all the time. I'd say 14 knots for the later clippers like Thermopylae of Marco Polo would be a good cruising speed in reasonable conditions.

Please do not express preferences for Hell over Melbourne in this thread. Those Cabbage Patchers are touchy sometimes.