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How to read the tire specifications. 

     In the example:  "120/70-17 ZR 58W",  the "120" represents the width (120 mm), "70" represents the profile aspect ratio (70% *see note), "17" is the rim size (17 inch rim), "Z" is the speed rating (Z means at least 149 MPH **see note), "R" is the type of tire (R= radial), "58" is the Load Index or Li (Front 58= 520 lbs, Rear 73= 805 lbs),  "W" is a supplemental speed rating (168 MPH).

 * note:  The "Z" speed rating of 149 MPH is the highest pre-1991 rating. Since then, tires designed with ratings over 149 MPH are manufactured with this marking plus additional "W" or "Y" markings as well. "W"=168 MPH, "Y" would be 186 MPH

**note   The profile aspect ratio is the ratio of the tire height to width in percentage. The height is the distance from the rim flange to the tread surface. Section height and width can be calculated by measuring the cross section of a tire.



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