Be more cautious on wet surface because even if it´s just drizzling the friction is reduced and you may also encounter water puddles that will create aquaplaning. So, better keep more distance to the one in front of you than you would normally do. Nice to have an M4 but oh well that´s out of my budget ... But my V60 still has a decent top speed of 225kph with 400Nm torque and I like to use that occasionally. Unfortunately around HD there´s too much traffic and too many speed limits / road works to allow for any meaningful high speed driving experience. My car has a "HD-" numberplate as well btw. - just not HD city ... I only enjoy fast driving when visiting my parents on the Baltic Ocean where I can even engage cruise control at 200kph for extended periods of time (frequently > 20mins at a time) on the A20 between Lübeck and Stralsund ... That´s fun. But despite we don´t have a general speed limit for the Autobahn there are simply too many speed limits in most areas to really have fun ... ... and for those not used to driving in Germany - all other roads do have a certain speed limit. All speed limits are enforced no matter where. There´s no difference between Autobahn and other roads other than the sanctions are stronger within city limits. Unfortunately we do have an increasing number of Autobahn speed limits that are not related to road safety but simply - as they say - to reduce noise. Usually these "environment related" limits are in the 120-130kph range but lower limits are common in a few densely built up areas and can be as low as 80kph.
I have driven in the rain on the autobahn quite a few times. Probably 4-5 thousand kilometers over a thirty year period in the rain. I have never seen ponding water on the autobahn like you commonly see in northeastern United States. The autobahn road surfaces are now porous which is designed to absorb water to prevent ponding. In a 2016 visit in Germany I did rent a VW Golf R from Berlin Tegel Flughafen. Was able to get the VW Golf R from Europecar. The roads are marked very well. Uberholen in der rechts Spuren einen Wagen in der linke Spuren fahren verbote.!! Die Straffen von Polizei for solute Verstoß absolut so teurer ist!
