I believe in old days ZR was for tyre speed rating over 240kmh(149mph), it has now been superseded by W & Y, in this case Y which is for up to 300kmh(186mph). ZR now is used for high performance tyres meaning they will probably be softer compound with better grip but increased wear and shorter life.
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Currently 8less
2011 Q7 S Line 3.0TDI, 2016 Tesla Model S 90D
8 history:
2006 A8 Sport 4.2TDI quattro SOLD,
1997 S8, reached end of life with gearbox failure
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