What is the maximum speed you are allowed to drive a bus in non-regular traffic outside built-up areas (not an autobahn or clearway for motor vehicles only)?

You are correct. In Germany, if you are driving a bus outside of built-up areas, but not on autobahns or special motor vehicle roads ("Kraftfahrstraße"), the maximum permitted speed is 80 km/h unless a lower limit is posted. This regulation helps ensure safety, as buses are larger vehicles and require more distance to stop, and many rural roads are not designed for higher speeds. Always watch for posted speed limits and adjust your speed to road and traffic conditions.