Like many facets of Italian life, the interpretation of road speed has two sides - theory and practice. In theory, speed limits are identical to other European countries - in practice everyone seems to drive about 10k over those limits and sometimes more. The carabinieri are quite strict about speed and in the event of an accident, excessive speed might invalidate your insurance. Most of all, an extra 10k might be the difference in killing another person.
Having driven many thousands of kilometres in hidden Italy, I would say that the majority of drivers are rational and well behaved. I would admit that there is a reasonable percentage of those who drive almost recklessly, but, it might be said, they are extremely implusive. You will find that those in a hurry will find a way around you (admittedly at vast expense to your coronory functions).
Overtaking is a necessary manouevre with most Italians and they generally obey the rules. However, beware of drivers undertaking on dual carriageways or autostrade (that is, passing on your right). They sometimes do this to avoid queueing past slower traffic such as articulated lorries.
It is generally a good idea to join the autostrada at a reasonable speed, depending on the road layout. Many entries do not allow speeds of more than 60k, due to the curvature of the road. Once you are parallel with the highway, you should try and achieve a reasonable speed. Similarily, beware that autostrate exits are often restricted in speed by a curved exit, so don't approach them at 100k when you need to have slowed to about 50k. Once you commit yourself to an exit, don't attempt to change your mind. It is usually possible to rejoin the road, simply by making an illegal u-turn before the toll booths