How to Read a Tyre Size: Sidewall Markings Explained

Anatomy of a tyre size

Here is the most common European passenger car size, decoded:

You may also see a "Z" inside the size (for example 225/45 ZR17) on older high-performance tyres; it simply means the tyre is rated above 240 km/h, and the speed symbol after the load index gives the exact rating.

From these numbers you can calculate the tyre's overall diameter: rim diameter plus two sidewalls. For 205/55 R16 that is 406.4 + 2 × 112.8 = 631.9 mm, giving a circumference of about 1,985 mm, or roughly 504 wheel revolutions per kilometre. Our tyre size calculator does this for any size in one step.

Load index: how much weight each tyre can carry

The load index is a code, not a weight. The table shows the most common values for cars, from tyre maker Pirelli's load index chart:

Never fit a tyre with a lower load index than the one specified for your car. A higher index is fine. The minimum is not linked to the car's kerb weight alone: it is set by the maximum permitted axle load, so heavy SUVs, vans and many electric cars need higher indices than their size suggests. You will often see XL (extra load) or Reinforced next to the size; these tyres carry a higher load index than a standard tyre of the same size and usually need higher pressures, so check the pressure table on the door pillar or fuel flap.

Speed rating: the letter after the load index

The speed symbol shows the maximum speed the tyre is designed for at its rated load.

Your tyres must normally match or exceed the speed rating in the vehicle documents, which is tied to the car's top speed. There is one common exception: winter and all-season (M+S) tyres may have a lower speed rating than the car's maximum, provided a sticker showing the maximum permitted speed is visible to the driver and you do not exceed it. Goodyear's sidewall guide spells this out, and rules on the sticker are strict in several countries, so check before you rely on it.

If you ever have to mix tyres with different speed ratings, for example after a puncture abroad, Pirelli advises fitting the higher-rated pair on the rear axle. Better still, replace tyres in axle pairs with the same model.

Other sidewall markings you should know

• Date code (DOT): a four-digit number at the end of the long DOT code, usually in an oval. The first two digits are the week, the last two the year: 0824 is week 8 of 2024. Tyres age even when unused, so check this when buying; many experts recommend replacing car tyres around ten years old regardless of tread.
• M+S: mud and snow. It is a manufacturer's own marking with no performance test behind it.
• 3PMSF (Alpine symbol): three mountain peaks with a snowflake. The tyre has passed a standardised snow grip test under UNECE Regulation 117. This is the symbol that counts where winter tyres are compulsory in Europe. See our guide to choosing winter tyres.
• Ice grip symbol: a mountain-like ice stalagmite. It marks Nordic tyres designed for ice and is tested under ISO 19447.
• TWI: tread wear indicator. Small arrows point to raised bars in the grooves. When the tread is worn down to them, it has reached 1.6 mm, the legal minimum in most EU countries.
• E mark (a letter E and a number in a circle): approval under UNECE rules for sale in Europe.
• OE markings: tyres developed for a specific car maker, such as AO or RO1 (Audi), MO (Mercedes-Benz), a star (BMW) or N0, N1 (Porsche). They are not compulsory, but some cars, especially sports models, are tuned around them.
• Max pressure: the maximum pressure the tyre structure allows, not the pressure to use. The correct pressure is on the car's label.

The EU tyre label is not on the sidewall, but it comes with every new car tyre sold in the EU under Regulation (EU) 2020/740. It rates rolling resistance and wet grip from A to E, shows external noise in decibels, and adds snow and ice symbols where relevant. A QR code links to the tyre's entry in the EU's EPREL database.

Which tyre sizes are allowed on your car?

The sizes your car may use are listed in its papers: the certificate of conformity (COC), and in many countries the registration certificate itself. The sticker inside the driver's door or fuel flap usually shows the main sizes with their pressures. Fitting a size that is not listed can require an approval or inspection in many EU countries and may affect your insurance, so always start from the papers.

Changing tyre size: the ±3% rule

People change size to fit larger alloy wheels, to get a cheaper winter size or to fit a narrower tyre for snow. The key is to keep the overall diameter close to the original, because the speedometer, ABS and stability control are calibrated for it. Tyre fitters generally keep the difference within about ±3%.

The first four are classic "plus sizes" of the same rolling diameter. The last one would make the speedometer read about 5% low: at an indicated 100 km/h you would actually be doing about 105 km/h. Wider tyres also change how the steering feels and can rub on suspension parts or the wheel arch on full lock. Wheel width, offset and the choice between steel and alloy wheels matter too.

Run any two sizes through the tyre size calculator: it shows sidewall height, diameter, circumference and the real speed at an indicated 100 km/h.