Tires: ZR vs R, Z Speed Rating Tire

Question
What is the difference between at P255ZR17 and a P255R17 tire?

Answer
Richard,

I don't think I can do better than what Tire Rack did:

http://www.tirerack.com/tires/tiretech/techpage.jsp?techid=35

Except:  The "Z" in the size is a way of telling folks that the tire can exceed 240 kph, but if there is a service description, that applies to both the speed and the load.

However, if the Service Description is not there, then the speed and the load are not defined in the usual way and you have to consult with the tire manufacturer.  (Hint:  If your vehicle is not capacble of exceeding 240 kph, then the load carrying capacity of a Z rated tire is the same as any other speed rated tire.