Ron,

These units like Stevo from Western Aust said would get along at around 55-60 mph, they all have 18 speed trans and use all the gears. A fairly common spec would be a double over 18 with 4.3 or lower diff ratios that would give them the ability to get up to cruising speed fairly quickly. A lot of the roads that these operate on you guys would laugh at, most commonly two lanes that would together be as wide as one lane on your interstates and when they get out in rural areas some dirt roads as well.
The accert Cat like in this rig are not very well liked ,they lost a lot of reliability over the single turbo c15's.

Mike.