on the autobahn, there's lots of stretches without speed limit. It is in fact the case though, that above 120kph you're not insured anymore. The 120kph is an 'advised' speed limit.
Close to the big cities and on ring roads, there will always be real speed limits, and they're frequently controlled. It's only in the more deserted parts where you can still give it the beans
by the way: the nurburgring nordschleife is officially a public one-way toll road without speed limit. It's not a track!