Tire Code - Explanation of Tire Codes

Explanation of Tire Codes

The ISO Metric tire code consists of a string of letters and numbers, as follows:

• An optional letter (or letters) indicating the intended use or vehicle class for the tire:-
  • P: Passenger Car
  • LT: Light Truck
  • ST: Special Trailer
  • T: Temporary (restricted usage for "Space-Saver" spare wheels)

Use of the letter P indicates that the tire is engineered to TRA standards and absence of a letter indicates that the tire is engineered to ETRTO standards. In practice, the standards of the two organizations have evolved together and are virtually interchangable.

• 3 digit number: The "nominal section width" of the tire in millimeters; the widest point from both outer edges (side wall to side wall). The tire surface which touch the road usually has smaller width.
• /: Slash character for character separation.
• 2 or 3 digit number: The "aspect ratio" of the sidewall height as a percentage of the total width of the tire. If the information is omitted, it is assumed to be 82% (if written, it should be like xxx/82). If the number is larger than 200, then this is the diameter of the entire tire in millimeters.
• An optional letter indicating construction of the fabric carcass of the tire:
  • B: bias belt (where the sidewalls are the same material as the tread, leading to a rigid ride)
  • D: diagonal
  • R: radial
    • if omitted, then it is a cross ply tire
• 2 digit number: Diameter in inches of the wheel that the tires are designed to fit. There is the rare exception metric diameter tires, such as the use of the 390 size, which in this case would indicate a wheel of 390 mm in diameter. Few tires are made to this size presently.
• 2 or 3 digit number: Load index; see table below
• 1 or 2 digit/letter combo: Speed rating; see table below
• Additional marks: See subheading below.