So what exactly is difference between r and zr tyres?

Black circles for example has pirelli zero neros at different prices

One is 84w xl but is zr17     w is speed rating I assume

Other is 84v but is only r17

so apart from the v/w speed rating whats the r/zr and why would you pick one rather than the other

[apart from speed rating that is]

just using neros as an example,was actually looking at vredestein ultrac cento zr17 w84 [XL?]

Answers are here.

 

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

 

To cut a long story short, you want the 84W rating - at least if you want to maintain the manufacturer recommended ratings. Over here at least, insurance (and maybe the law as well) requires at least the same speed rating on replacement tyres.