paul b:
think the point was on A roads such as we have here, i.e 60 for cars 40 for us.
There are roads in Germany (called B roads) that are the equivelent of our A roads, that do have differing limits i.e. 100kmh for cars, 80kmh for trucks.
are they single carriageway?
Single carriageways in France and Belgium also have different limits for trucks and cars.
There are a lot where there are limits as low as 40kmh for trucks while cars can do 70kmh.
paul b:
are they single carriageway?
Yes, although the German road designers were considerate/sensible* (*delete as appropriate) to put an overtaking lane on alternate sides every 5-10kms or so, thereby reducing the occurences of the ‘suicidal car driving overtakers’ syndrome!! 
wonder if you took a poll of say 100 members of the general public, how many would know that hgv’s are restricted to 40 on our single A roads? my guess would be about half a dozen if that.