[Dkennel]

Actually the "recommended" speed is 130km/h, Autobahn Richtgeschwindigkeit. Many areas have posted speed limits due to visibility, congestion, etc... Several years back the State of Hessen had speed limits on all it's Autobahns, not sure that is still the case.
I was just back in January and there were plenty of stretches without speed limit. You can supposedly loose your insurance coverage, if the carrier can show you were reckless and speed was inappropriate for weather, road conditions, etc...
Not a lot of deserted parts in Germany, the biggest problem is traffic density, congestion, and construction zones...