There is an agreement between the German premium car manufacturers to limit top speed to 155mph (250kph)...at least 'out of the box'.

Porsche didn't bite though.
Various reasons have been suggested for this move over the years. Some say it is to appease the left-wing parties in Germany over safety and environment concerns, others say its to do with the technical limitations of tyres and aerodynamics.
There is also a similar agreement between the Japanese bike makers to limit top speed to 187mph (300kph for reason being that it is nice round figure like 250kph).
Its no secret in enthusiast circles that some of these high performance cars are electronically limited to 155mph when - derestricted - the true top speed is much higher. It's equally well known that these speed limiters can be bypassed with a little bit of tweaking. I understand Audi (I think) will even do it 'for a fee' on some models.