100 in general for trucks and 130 for cars
70 or 80 max on the C-D road for trucks and 110 for cars
The server says "simulation" but in my experience many drivers just want to "simulate" how fast they can race to their destination which just breaks immersion.
Also I guess many people enjoy the C-D road for how many people there are but get bothered by those "racers" that seem to mistake ETS2 with Shutoko.
This limitation might reduce collisions there that are often caused by speeding/overtaking.