The speed limits in Germany are 50 km/h (50 mph) in built up areas, 90 km/h (56 mph) outside built up areas, and 130 km/h (86 mph) on motorways.
The motorways are are called Autobahns and are just numbered. They are also toll free. The non-motorway main roads are also just numbered, and the smaller roads just have road names.
Certain parts of the Autobahns have no speed limit and when driving on these roads it is very important to take great care when overtaking a slower moving vehicle as some other vehicle may be approaching you at extremely high speed. The best advice that I can give is once you decide to overtake make sure nothing is coming up fast behind you and then once you have made your ovetake get back in to the inside lane as quickly as possible.
The motorways signs are in Blue, the main road signs are in Yellow, and signs for the smaller roads are white.
All of the roads in Germany are very well maintained.
There are a lot of speed cameras and patrolling police cars everywhere in Germany.
Petrol was generally 20 cents more expensive per litre on the Autobahns and at the time of writing the cost elsewhere was 1.60 per litre.
My favourite German roads are in Bavaria in the Black Forest area.
I did not buy a SIM card for a mobile phone whilst I was in Germany so I have no information about how to do this.