Some odd replies on here ! I lived for 7 years in Germany and the autobahns are no different to uk motorways firstly. Most have speed restrictions and easy to read signage. unless stated otherwise most are 130kph (81MPH) .
the deristricted sections are normally longer rural stretches away from cities and are marked with a black line through a circle, very similar to our national speed limit sign.
it states in the highway code that you can drive at a speed where you are in control and in conjunction with the road conditions and traffic levels. (unless signs state otherwise)
One of the best signs to remember on fast roads in STAU (traffic jam) these can be a few KM early when the road is a fast one and remember that static traffic can come up on you at a rapid rate.
contrary to popular belief you cant go as fast as you like in Germany. (although bike blasts quite commonly clipped 150+