I've been fined on German highways for going too fast. Memes like these are myths. It is based on fact that there is no default upper limit on highway. But there are speed-limit signs on German highways.
I think it's based on the (fahrt fahrt fahrt auf den) Autobahn. Now, I'm not german so I could be wrong, but here's what wikipedia says regarding speed limits on the autobahn:
Much of the system has no speed limit for some classes of vehicles.[1] However, limits are posted and enforced in areas that are urbanised, substandard, accident-prone, or under construction. On speed-unrestricted stretches, an advisory speed limit (Richtgeschwindigkeit) of 130 kilometres per hour (81 mph) applies.[2] While driving faster is not illegal in the absence of a speed limit, it can cause an increased liability in the case of a collision (which mandatory auto insurance has to cover); courts have ruled that an "ideal driver" who is exempt from absolute liability for "inevitable" tort under the law would not exceed Richtgeschwindigkeit.
So, it certainly seems there is a basis for the jokes, even if they aren't 100% historically accurate.
I've driven around the Western side of Germany a few times and there really weren't many sections that were unrestricted, and those that were unrestricted were not that long, and often very busy.
Anecdotally, I did ask about it the first time I went and the advice I got was that while it is technically unrestricted you should be going at a similar speed to the traffic around you. If there are people in the middle lane and you blast past them at double their speed, that could be seen as unsafe by police and get you in trouble.
You can totally go 250km/h on the A5 from Frankfurt to Darmstadt, if traffic allows it. That wont always be possible, but typically if it isnt during work traffic, 160-200km/h is doable comfortably, even on other parts of the Autobahn
The longest unrestricted part that ive driven should be the A71, where you can go longer passages without restrictions, save for the tunnels.
An Autobahn has per default no speed limit for cars, except when there is one indicated by a speed limit sign. But there are many parts on any Autobahn without speed limit signs, so in reality you can very often drive as fast as you want.
There are many opportunities to drive with 200km/h or more if you want, but often the traffic prevents going with this speed for longer than a few seconds or minutes.
Plus, anyone who has traveled in South Asia or South America knows that Germany is much, much more safe to drive in than most of the rest of the world.