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Germany is by far my most favourite country for driving in. Even their 'B-roads' are amazing. But be aware that not all autobahns have unrestricted speed limits throughout their length. Lane discipline is far better there than in the UK too where centre lane hogging is shocking!
You might be cruising along at 120mph+ but watchout in your rear view mirrors for very fast Audi's, Mercs, Porsches, BMW's, etc which are doing 160mph+. That's where DRL's really help (and what prompted me to get mine).
I've been to/through Germany lots of times and the GTI, even when she's been loaded with 3 people and luggage, has been amazing. She is modified though.
Also, a town name sign is automatically the start of a speed limit zone and the same sign with a red diagonal stripe across it signifies the end of both the urban boundary and the end of the speed limit. There's much less signage. Respect and obey it and don't take the piss.
Oh, and be very careful when heading back through northern France - The French police love to catch English cars for speeding!
When you hit the M20 in the UK again 70 will feel like 40 - It's very very dull