Tyre Size Designation

What is tyre size designation and how to read it? The tyres are generally specified and designated by the nominal size of their sectional width and the wheel rim diameter.

The cross-bias ply tyres are generally designated as: 7.5 x 14 x 6PR

The designation means that:

1. The section width or thickness of tyre from shoulder to shoulder is 7.5 inches.
2. The diameter of the wheel rim is 14 inches.
3. PR means ply rating. This represents the maximum recommended load which the tyre can carry when used in a specific condition. It is an index of tyre strength and does not represent necessarily the number of cord plies in the tyre.

The radial tyres are generally designated as 145/70 R 12 69 S

This designation means that:

1. The section width or thickness of tyre from shoulder to shoulder is 145 mm.
2. The aspect ratio is 70%. The aspect ratio is defined as the ratio of section height to section width of the tyre. In this case, it means, the section height is 70% of section width i.e. 101.5 mm.
3. R represents that the tyre is radial.
4. The diameter of the wheel rim is 12 inches.
5. The load index is 69. The load index defines the maximum load (mass in kg) a tyre can carry under specified condition.
6. The speed symbol is S. The speed symbol defines the maximum speed for which the tyre is rated to carry a load corresponding to its load index, under specified conditions.