Okay, I have it all figured out now (actually I figured it out a while ago, as a result of poking around the estabs and with some help from Arjuna, but I haven't got around to posting it.
It seems like all units are given a typical upper move speed limit based on their characteristics:
9/6 kph for foot units
50/38 kph for most motorized units (wheeled or tracked, and typically line units but also SP arty)
20/30 kph for certain motorized units, primarily ones like bases that seem like they would have
more control or other difficulties with moving around. Also includes towed artillery.
60/40 kph for motorized recon units.
These are all upper limits on speed that I think are probably based on the traffic control issues discussed above. In the estab editor these values are set in the Force Editing window for each force.
However the movement speed of a unit is further limited by the actual maximum speed of any vehicles it has. Since the vehicles have both a road speed and a cross country speed, their actual movement speed may vary depending on the terrain. Basically if any vehicle in a unit can't keep up the whole unit moves as fast as the slowest vehicle.
However since the movement type to be used (foot, mot, tracked, horse, bicycle, etc) is specified by the force, not by the vehicle, I'm guessing that any modifiers to speed based on that are applied to the force speed limits, not the vehicle speed limits.