Read a tyre sidewall marking
Paste what is written on the sidewall (205/55 R16 91V XL M+S) and get the real diameter, the load it carries, its top speed, the meaning of every marking and the age of the tyre from the DOT code.
Everything is worked out inside your browser: what you type is never sent anywhere.
How to read 205/55 R16 91V
The first number is the section width in millimetres. The second one is not a measurement but a percentage, the aspect ratio: 55 means the sidewall is 55 per cent of the width, so 112.8 mm. The R stands for radial construction and 16 is the rim diameter in inches, that is 406.4 mm. The overall wheel diameter is the rim plus twice the sidewall, 631.9 mm. Then come the load index (91) and the speed symbol (V). Mistaking the aspect ratio for millimetres is the most common error, and it is also why two tyres with the same width can end up with wildly different diameters.
Where the kilograms and the speeds come from
The load index and the speed symbol are not free numbers, they point to two tables fixed by the tyre standards (ETRTO, taken up by regulation ECE R30). Index 91 means 615 kg per tyre, index 100 means 800 kg, and the scale ends at 130, that is 1,900 kg. Among the speed symbols, T means 190 km/h, H means 210, V means 240, W means 270 and Y means 300. A Z before the R (225/40 ZR18) is the old marking for tyres above 240 km/h, and where no final letter follows it still applies. The kilograms hold at the design pressure: an underinflated tyre carries far less than its index claims.
The DOT code tells you the real age
The last four digits of the DOT code are the week and the year of production: 2318 means week 23 of 2018, so early June 2018. The four digit format has been in use since 2000; before that it was three digits, and a tyre that old belongs in the bin regardless. The date you get is the start of that week, so it is accurate to within a few days. No law sets an expiry date for a tyre already fitted, what gets checked is tread depth, which must stay above 1.6 mm across the European Union (directive 89/459/EEC). The advice to replace tyres at around ten years comes from manufacturers rather than from a rule, which is why this page states it as advice.
Before buying, compare it with what your car asks for
The registration document or the door placard lists one or more approved sizes with their indices. The tyre you fit must have a load index and a speed symbol at least as high, never lower, and that is why this page lets you type the indices you need and answers straight away. The usual exception concerns winter tyres, which in several countries may carry a lower speed symbol, down to Q, provided a sticker inside the cabin shows the maximum speed to keep. This page does not replace your tyre fitter or your registration document: it reads the marking, not the approval of your particular car.
What this page cannot do
It only reads the text you give it, so it has no idea whether the tyre is crazed, repaired or has spent three years in the sun. The millimetres it works out are geometric: under the weight of the car the rolling circumference is roughly 2 to 3 per cent smaller, so if what you want is the speedometer error when moving from one size to another, use Tyre size comparison, which is built for that. The MAX PRESS stamped on the sidewall is the limit of the casing and not the pressure to run: the right one is on the door pillar placard. No data leaves your device.