Just came across this thread. Maybe I can clarify some things.

- There is no general speed limit on the autobahn. The above mentioned 130kph are a general suggestion for save driving when there is no speed limit. IT is a sign with white letters on blue ground.

- If there is a speed limit it's always marked with a red circle and black (regular sign) or white letters (digital sign). Escpecially around Stuttgart are most parts of the autobahn regulated by a traffic control system which includes digital signs that vary the speed limit according to traffic and weather conditions.
Be aware that most of these digital signs have integrated speed cams!!

- In general for driving on the autobahn: You're not allowed to overtake the car in front of you on the right (should be the same in spain). If you're driving on the very left lane make shure to watch you're mirrors - even when you're going +180kph. There are a lot of Porsche around Stuttgart...

- Fast driving on the autobahn directly around Stuttgart is nearly impossible - heavy traffic, many construction going on. The A8 to munich is a PITA to drive - except for very late in the evening or in the night. You should try the A6from Heilbronn to Nuernberg or the A81 from Heilbronn to Wuerzburg.

And if you're around munich write me a PM