The first number in the description represents width, expressed in millimetres, calculated by measuring the greatest distance from flank to flank. Before this measurement can be taken, the tyre must be mounted onto the rim, and should not bear any weight during the process. In the case of the 205/55 R15, cross section width is 205 mm. Small, tolerable variations may occur during the production process, therefore actual width may vary slightly for individual tyres. For this reason, standard sizes are separated by 10 mm intervals.
Following the width indicator, the second number specifies the aspect ratio, being in fact a percentual value. Are you unsure of what this represents? Aspect ratio is simply a measure of cross section height, in relation to width. Therefore, the exact flank height remains to be determined. In the case of the 205/55 R16, the flank height is equal to 55% of 205 mm. Thus, the tyre has a cross section height of 112.75 mm. Tyres that display a value smaller than 50% are referred to as low aspect ratio tyres, while an aspect ratio of 80% is considered high.
The third number in the description specifies the tyre's inner diameter, a mandatory inscription that appears not only on the sidewall, but also in the registration certificate. This makes clear what size of tyre is suitable for use with a certain type of vehicle. To be certain of what the correct rim size for any tyre is, we must look at the final number in the description – it indicates the tyre's inner diameter and, equally, the overall rim diameter, expressed in inches. Of course, you can purchase complete wheel and tyre sets, and you need not worry about matching rims and tyres any longer.