Michelin Pilot Super Sport question

[babydriver]

Here's one for the tire experts.

What is the difference between these two tire designations for the Michelin Pilot Super Sport?:

215/45R17XL 91Y BW


Specifications:

• Traction Grade:
  AA
• Temperature Grade:
  A
• Load Index:
  91 1356 lbs.
• Speed Rating:
  Y 188 mph
• Tire Size:
  215-45-17
• Performance Grade:
  Ultra high performance
• Treadlife:
  30,000 MILE
• Tread Depth in inches:
  10/32
• Treadwear Grade:
  300

215/45ZR17


Specifications:

Serv. Desc: (91Y) Load Index 91 = 1356lbs (615kg) per tire
Speed Rating “(Y)” = 186+mph (300kph)
Load Range: XLMax Load = 1,356 lbs
Max psi = 50 psi

UTQG: Treadwear: 300
Traction: AA
Temperature: A300 AA A




I note that there is a Z in the second one but not in the first. Otherwise I see no difference. What does the Z tell us?


Thanks.

[N1rve]

Z tells you the speed rating.

Z is 149 MPH and over. (No max speed indicated)

The first one (XL) is extra load or reinforced and are Y rated tires (up to 186 MPH).

[DM7]

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

Part way down the page the Z in the middle of the tire size code is explained. Tire people thought that tires would not need specific ratings for 'over 149MPH', so they choose the last letter in the alphabet. Since then there has been a need for specific speed ratings for 'over 149MPH' so other characters in the alphabet are now used for speed ratings higher than 149MPH. Including the Z in the size code is considered an antiquated practice and is just a quick way to show that a tire will do more than 149MPH.

Both descriptions you have listed are Y rated.

Tire rack has a great learning section on their site. Now you too can be the tire expert.

[babydriver]

Originally Posted by DM7

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

Part way down the page the Z in the middle of the tire size code is explained. Tire people thought that cars would not need tires that will drive over 149MPH, so they choose the last letter in the alphabet. Since then tires have increased in speed rating and so other characters in the alphabet are now used for speed ratings higher than 149MPH. Including the Z in the size code is now considered an antiquated practice.

So the Z is superfluous; good information, thanks!