Tyre size explained: how to read your sidewall

Reading 225/45 R18 95W from left to right

Read left to right and the size tells you the tyre's width, the height of its sidewall, how it is built, the rim it fits, what it can carry and how fast it is rated. The sidewall on this example stands around 101 mm tall, because an aspect ratio of 45 means 45 per cent of the 225 mm width.

Some sizes carry extra letters. A P indicates a passenger-car tyre to North American sizing, LT means light truck, and ZR in place of R appears on many high-speed fitments. XL, Extra Load or Reinforced denotes a stronger casing rated for more weight at a higher inflation pressure. Compare the whole string when you shop, not just the first three numbers.

• 225 is the section width in millimetres
• 45 is the aspect ratio, the sidewall height as a percentage of the width
• R is radial construction, used by almost every modern car tyre
• 18 is the rim diameter in inches
• 95 is the load index, read from a published load table
• W is the speed symbol, read from a published speed table

Width and aspect ratio change more than looks

Width and aspect ratio together set the tyre's overall diameter, and that diameter affects speedometer accuracy and whether the tyre clears the suspension and bodywork at full steering lock. As a general rule, a wider tyre on a shorter sidewall gives firmer ride quality and more direct steering, while a taller sidewall is more forgiving over broken edges and kerbs.

So changing size is not a free choice. Ask for the overall diameter of any alternative to be compared with the original, and check the handbook for the sizes the manufacturer approves. A diameter far from standard, or mixed sizes across one axle, can affect braking and stability systems and, on four-wheel-drive vehicles, the drivetrain.

Load index and speed rating are limits, not suggestions

The load index is a code rather than a weight. It is read from a published table, where 95 commonly corresponds to 690 kg per tyre, and the speed symbol works the same way, with W commonly listed as 270 km/h. Both describe what the tyre is certified to do under test conditions at its correct pressure. A speed symbol is not a recommended cruising speed, and it means little on a tyre that is underinflated, overloaded or damaged.

The practical rule is straightforward: do not fit a lower load index or a lower speed symbol than the vehicle manufacturer specifies. Cars that carry heavy loads, tow, or run stiff low-profile fitments often call for the Extra Load version of a size. If an alternative is offered, ask how its load index and speed symbol compare with the placard figures.

The DOT code tells you when the tyre was made

On the sidewall you will find the letters DOT followed by a string of characters. The last four digits are the date code: the first two give the week of manufacture and the last two the year, so 2425 means week 24 of 2025. It is worth asking to see this code before you buy, because tyres can spend a long time in storage before they reach a vehicle.

Age matters alongside tread depth. Heat, sunlight and ozone change rubber over time whether or not the car is driven, so an old tyre with deep tread is not automatically a sound tyre. Follow the tyre maker's guidance and any applicable local requirements on age, include the spare in your checks, and replace sooner if you find cracking, cuts or bulges.

Other sidewall markings worth knowing

The maximum pressure moulded on the sidewall is the marking most often misread. It is a ceiling for the tyre as a component, not a recommendation for your car. Your correct cold inflation pressure comes from the vehicle placard or handbook, set for your car's weight, suspension and tyre size.

• Max load and max pressure: limits for the tyre, not targets for your vehicle
• M+S, All Season or a mountain and snowflake symbol: intended-use markings
• Rotation arrows or an Outside marking: directional or asymmetric tyres that must face a particular way
• Tread wear indicator bars in the grooves: the reference point for replacement

Staggered sizes and where to find your placard

Many rear-wheel-drive and performance cars use staggered fitments, with wider tyres on the rear axle than the front and occasionally a different rim diameter too. The two sizes are not interchangeable, so quote both whenever you ask about tyres and make sure the fitter knows which size belongs on which axle.

The reference for your vehicle is the tyre information placard, commonly a sticker in the driver's door jamb and sometimes inside the fuel filler flap, on the glovebox lid or in the handbook. It lists the original front and rear sizes, the recommended cold pressures, often a separate pressure for a fully loaded car, and the spare. The tyres currently fitted are not proof of the correct size, because a previous owner or fitter may have changed them.

Photograph the placard and the sidewall of each tyre and you will have everything a fitter needs. If a marking is hard to read, or the fitted sizes do not match the placard, the team at Wagon Wheel in Salmabad can check what is on your car and talk through the options.