Hi @MichiSchmid,
there are a lot of things that get taken into account when calculating the speeds used for travelling, especially for cars. In general though, any speed limit applied on the way in OSM is given priority, and then if that isn’t available then heuristics are used to generate speed such as the road type, surface material etc. Other code is then applied in a number of instances to try and reduce the speed to take into account that you are not travelling at full speed all of the time, but that is currently under review and likely to change soon.
To get a general idea though, there is a JSON file that defines some default speed limits when the information is not provided in OSM: