What is the longest stretch of the Autobahn that is guaranteed to have no speed limits?

I can not comment yet on this SE, and this is not an answer to the question, but I feel like I have to warn you. And the other two answer you got (Jan's and Tor-Einar Jarnbjo's) are still pretty good.

Before you go racing, you may want to check this video about a situation identical to yours:

https://youtu.be/fUapNMXiz0k

This happened a year ago on a Hungarian highway. There are absolute speed limits in the country, but the guys did not care... The initial situation is pretty much like what you planned: they rent a Lamborghini Huracan, took it on the highway, and drove it at 330kmph., while the passenger was recording. The highway there is a really high quality one, as close as you could get to a raceway in an Autobahn. (As a Hungarian I have been there occasionally.) Still, the event happened:

They lost control of the supercar, It wrecked, turned upside down and burned completely out. The driver suffered, miraculously, only light injuries, could get out of the car and pull the seriously (permanently!) injured passenger before it burned to ashes, thus they survived the impossible. The recording they made also survived, making it possible for the Hungarian Police to submit it.

The moral is that you must be extremely careful when you do what you planned. This was an identical situation, and the result was nearly fatal. And considering most of the German Autobahns were built long before this part of highway, they might be in worse condition, making this outcome more likely.

I've had the (mixed!) pleasure of racking up many, many kilometers on German highways, also in various cars that can go faster than most other traffic. While I have no specific answer to what stretch is best, please note several caveats that might, in the worst case, get someone killed:

• You should not go "too" fast in a car that you are not familiar with. E.g., how well does it really brake; how well does it really steer when you're braking hard; how skilled are you at high speed driving, and high speed crash avoidance?

• If you're not used to driving above your national speed limit (130kmh in your country) then even driving "only" 170kmh is going to feel fast and it will be exhausting after a fairly short while. With experience, one can drive at 180kmh for a few hours, but it's still going to be exhausting. Driving at 230+ kmh is incredibly exhausting, even after only 15-30 minutes.

  • Note that "exhausting" doesn't mean that you start to sweat or grow physically tired. But you very quickly develop extreme tunnel vision as well as mental fatigue. This will severely reduce your ability to react appropriately in dangerous situations.

• Don't overdo it. If you find yourself on a sweet stretch in perfect conditions, remember that even roller coasters go slow sometimes. Don't push the car beyond your(!) abilities just to see how far beyond 270kmh it will go. A "nice sports car" from Germany will probably be electronically limited at 250kmh but don't try to find out, certainly not on your first drive.

• The stretches mentioned by others are only what the maps say. There can very easily be road works or other local circumstances that can put up unannounced limits.

• Experience (others' and yours) might tell you that a certain stretch is fairly empty, but there is no guarantee against other idiot drivers, or big lorries, long or sharp curves, or other reasons to slow down.

• Collision avoidance becomes much more difficult if there are only two lanes, as opposed to three lanes or more. If you're going to go fast, don't ever do it in a two-lane section (yes, two-lane stretches without limit exist).

• Even if you would be driving the safest car in the world, and you would be the best driver in the world, don't ever count on having the road to yourself.

  • At high speeds, you can very quickly rush up on another car that you hadn't noticed.
  • Other drivers might become spooked by your speed (you weren't in their mirrors just seconds ago!) and they might do stupid things; out of surprise or even out of provocation!
  • There can be animals, objects, or side wind.

As a personal anecdote, one situation remains clear in my memory. I was driving in the middle lane in a regular car at around 160kmh when I noticed a fast car approaching in my rear-view mirror. From the moment I noticed it and until it passed me and got out of sight, only seconds had elapsed. If you ask me to describe the car, I will say it was red, it was about a meter high, it looked to be fifteen meters long, and it was blurry. I haven't done the math but whatever speed he was going, it was not safe in that traffic.

_{(Please excuse this bit of soapboxing. I'm not trying to talk Jonathan out of his trip but I feel it is prudent to highlight some safety aspects. Driving fast can be fatal to others, not just the driver.)}

There is absolutely no part of any German highway guaranteed to have no speed limits. Even if there is usually no speed limit, road works are quite common, not always announced in advance and usually bring with them a speed limit of 80 or even some time 60 km/h.

If you still want to give it a try, you can find several helpful resources online. The web site http://autobahnatlas-online.de/ (unfortunately only in German) has several maps with information about the German highway network. You can find a map of all highways with their speed limits here and an explanation of the colours here (highways without speed limits are dark blue).

My persional suggestion would be to try the eastern part of A20. Between the junction 'Uckermark', the eastern end point of the highway and Jarmen, you have about 100km without any regular speed limits. After a short stretch with 100 km/h where the highway goes through the city of Jarmen, you have another long strech of highway without any speed limits. The A20 is also one of the highways in Germany with the least traffic. I have only driven the suggested eastern part a few times, but even in the daytime, there was almost no traffic on the road.

A few disadvantages is of course that this particular strech of highway is in an area without much tourism, there is not incredibly much to see and even the scenery is rather boring.

As someone who has driven well past 200 km/h often, I can offer these additional suggestions, based on my own experience:

1. You need to concentrate extremely hard at any speed above 150 km/h, because you have very little reaction time.

2. DO NOT ATTEMPT THIS IN A CAR YOU ARE NOT FAMILIAR WITH. I cannot stress this enough.

3. Make sure you are using the correct type of tires and they are at the correct pressure for the make and model of the car. At high speeds, the intense heat can warp the shape of the tires (that's why they are speed rated); and it can affect everything from handling, steering response and most importantly, braking response. Driving fast in a car with tires that are rated only at 120 km/h is looking for disaster.

4. It is very unlikely you will get a clean stretch of highway for very long (at speeds above 150 km/h). So you are risking a lot for a few minutes of thrills. You'll enjoy it better if you take your car to a public racetrack.

5. Even if you are the best driver in the world, you are on a public road with all kinds of traffic and all kinds of drivers. Practice defensive driving.

There is actually a South African company that provides what they call Autobahn Blasts: driving at high speed on a de-restricted section of the Autobahn.

Now, I do not know if they're trustworthy, but they seem to have a number of high-profile sponsors. Here's a link to the relevant page on their website:

http://www.roadandrace.co.za/services/road-tours-around-the-circuit/