Specific dimensions, such as 205/65 R15, are standardised and displayed on all tyres of that size category. The first number refers to width, expressed in millimetres, and thus a 205/65 R15 tyre is 205 mm wide. To obtain this measurement, the tyre must be mounted upon the rim and it must not bear any load. Then, the distance between the two most distant points on either flank is measured. Therefore, width so expressed is not a measurement of the rolling surface, and is must also be noted that this is an approximate value. This is largely owed to minor variations that occur during the production process.

15 inch radial tyres

The second value in the inscription can be used to determine the height of the tyre's flank. For this, first we need to know the width, which we do, as it is indicated by the first number. We also need the second number, indicating aspect ratio, which is a percentual value that we then multiply by the width value. In the case of a 205/65 R15 tyre, the aspect ratio is equal to 65% of 205 mm, resulting in 133.25 mm; do note that here, as well, certain tolerances are admissible. A 65% aspect ratio is neither small, nor is it large, as an 80% aspect ratio would be. The letter R stands for radial tyre, a specific constructive type that has superseded the diagonal type on the market. This is followed by yet another number – here, 15, indicating the tyre's inner diameter, in inches.