The size on the tyre sidewall, explained figure by figure

On the sidewall of every tyre there is a code that looks like a code and is in fact a list of measurements. Knowing how to read it avoids buying the wrong one.

205/55 R16 91V, piece by piece

• 205: the tyre width in millimetres.
• 55: the aspect ratio: the sidewall height is 55% of the width, so about 112 mm. It is not an absolute value: if the width changes, the height changes too for the same number.
• R: radial construction. Practically all car tyres are.
• 16: the rim diameter in inches.
• 91: the load index: 91 corresponds to 615 kg per tyre.
• V: the speed rating: V is valid up to 240 km/h.

What you can change and what you cannot

The permitted sizes are the ones in the vehicle registration document (or in the manufacturer's documents). The overall wheel diameter must stay practically the same: it determines the speed shown on the speedometer and the operation of ABS and stability control.

That is why the documents often list different sizes with almost the same outer diameter, such as 205/55 R16 and 225/45 R17: a wider tyre with a lower sidewall.

The load index and speed rating cannot be lower than those prescribed. For winter tyres some countries, Italy included, allow a lower speed rating under certain conditions: check the rules of the country you drive in.

If a size is not in the registration document, it is not "almost the same": it is a different size.

The markings that matter after the size

• M+S: mud and snow. It is a statement by the manufacturer.
• Snowflake inside the mountain (3PMSF): the tyre has passed a snow test. It is the marking that counts for many winter requirements.
• XL or Reinforced: reinforced construction, higher load index for the same size.
• RunFlat (RFT, ROF, ZP…): self-supporting sidewall, you can keep driving on a flat tyre for a limited distance, at the speed and distance given by the manufacturer. Fit them only on cars designed for them, and never together with normal tyres on the same axle.

Age, which matters more than it seems

On the sidewall there is the marking DOT followed by four digits: `3223` means week 32 of 2023. A tyre ages even when unused, because the compound hardens over the years and grip gets worse even if the tread is intact. Manufacturers say after how many years to have it checked and when to replace it in any case.

A "new" tyre made years ago is new only in the box.

The minimum tread, and the useful one

In Italy and the rest of the European Union the legal minimum is 1.6 mm; some countries require more for winter tyres. It is a limit, not a recommendation: with less tread the braking distance in the wet gets longer and aquaplaning starts at lower speeds, and below 3 mm it is already worth thinking about replacing them.

The tread wear indicators (TWI) are the raised bars inside the grooves: when the tread is level with them, you are at the legal limit.

This advice is general: for the values and procedures for your car, refer to the manufacturer's handbook.