Understanding and calculating tire size codes
Formula, meaning and practical tips
The tire size code follows a Europe-wide standard. Take the example 205/55 R16: the first number (205) is the tire width in millimeters. The second number (55) is the aspect ratio, i.e. the sidewall height as a percentage of the width. R stands for radial tire (today's standard), and 16 is the rim diameter in inches.
The outer diameter is calculated from the rim diameter and the sidewall height: outer diameter = rim (in mm) + 2 × sidewall. For 205/55 R16, the sidewall height is 205 × 0.55 = 112.75 mm. The rim diameter is 16 × 25.4 = 406.4 mm. That gives: 406.4 + 2 × 112.75 = 631.9 mm outer diameter.
The tire circumference is decisive for the speedometer. It is calculated as π × outer diameter. At a diameter of 631.9 mm, this gives a circumference of around 1,985 mm. This means: at a vehicle speed of 100 km/h, the wheel rotates about 14 times per second. If the tire size deviates, the speedometer shows an incorrect value – which is relevant both for safety and for fines.