As stated before, not every part of the Autobahn is without speedlimits. But I think most parts of it are. I have traveled many times on it, as a passenger, as I have no driver's permit. And of course, the best times were when we were, like, the early morning hours, around five in the morning, and in a well-powered Mercedes Benz rental car that my colleague who drove it, got up to 210 km/h (60 miles per hour are 100 kilometers per hour, you do the math
)
Of course, he brought along a burnt CD with speed meatal tracks. That was one occasion I enjoyed metal!
And yes, the german autobahn is in a condition like runways, partially because they were planned as backup runways in case of war, for the allied forces back in the day.