Not sure if you will actually do any driving in Germany, but here the speed limits are posted everywhere, but as a general rule, it's 50 in villages, 100 on open roads, and no limit on the autobahn unless otherwise posted (iirc 130 is recommended). If you see the sign below, it's no limit, but more often than not, especially in the area where I live, there is a posted limit.
Be careful for speed cameras, though - they're everywhere. And don't drink and drive (although that should be obvious). The Germans are really strict about that.