ZR and R tires are the same, radial in construction. The actual sp rat of the tire will be found after the size. Example, 245/45ZR18 98Y, Y being the physical sp rating of the tire.

Check out our links on sp ratings and sidewall markings here Sidewall Markings Speed Rating, Load Index & Service Descriptions
As far as weather conditions, again, the a/s ability has a bit more to do with the tread design and compounds than the speed rating.