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Actually if I remember right a ZR in the tire size description actually means there is no speed rating(above 149mph) There is no actual Z rating anymore Y is the highest then add the ZR to the width description..... Here, look at this and see if it helps!!
SPEED
SYMBOL MAXIMUM SPEED APPLIES TO
PASSENGER CAR TIRES APPLIES TO
LIGHT TRUCK TIRES
* above 149 mph (240 km/h) YES -
Y** 186 mph (300 km/h) YES -
W** 168 mph (270 km/h) YES -
V (with service description) 149 mph (240 km/h) YES -
H 130 mph (210 km/h) YES YES
U 124 mph (200 km/h) YES YES
T 118 mph (190 km/h) YES YES
S 112 mph (180 km/h) YES YES
R 106 mph (170 km/h) - YES
Q 99 mph (160 km/h) - YES
P 93 mph (150 km/h) - YES
N 87 mph (140 km/h) - YES
M 81 mph (130 km/h) YES
Temporary
Spare Tires -
*For tires having a maximum speed capability above 149 mph (240 km/h), a "ZR" may appear in the size designation. For tires having a maximum speed capability above 186 mph (300 km/h), a "ZR" must appear in the size designation. Consult the tire manufacturer for maximum speed when there is no Service Description. Example: P275/40R17 93W at 168 mph (270km/h) or P275/40ZR17 at above 149 mph (240 km/h). Consult tire manufacturer.
**A "ZR" may appear in the tire size designation.
NOTE: For "V", "W", or "Y", and tires with a "ZR" rating, a vehicle load adjustment (for speed) is required; consult tire manufacturer.
Source: Rubber Manufacturers Association
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