Tyre Sizes UK: How to Read Your Tyre Sidewall Markings

A tyre sidewall looks like a jumble of numbers and letters, but almost all of it is standardised and each part means one specific thing. Once you can read it, buying replacements, checking a used car has the right tyres fitted, or working out whether a set of wheels will fit becomes straightforward.

The most common UK format looks like this: 205/55 R16 91V. Here is what each element is telling you.

How to read a tyre size

Read a tyre size left to right as width, profile, construction, rim diameter, load, speed.

A lower aspect ratio means a shorter, stiffer sidewall. A 40 profile tyre on a 19 inch wheel has far less rubber between the rim and the road than a 65 profile on a 15 inch wheel, which is why bigger wheels ride harder and are more vulnerable to pothole damage.

Load index

The load index is a code, not a weight in kilograms. It refers to the maximum load one tyre can carry at its rated pressure.

Never fit a tyre with a lower load index than the manufacturer specifies. This matters most on vans, seven seaters, estates carrying loads, and electric cars, where the battery pack pushes kerb weight well above the equivalent petrol model.

Speed rating

The speed rating is the maximum speed the tyre is approved to sustain while carrying its rated load.

Speed ratings are not an invitation, and UK speed limits apply regardless. They matter because the rating reflects how the tyre is built to handle heat and load, and because fitting below the manufacturer's specification can affect insurance and can be flagged at MOT. The recognised exception is winter tyres, which may carry a lower rating provided a label showing the maximum permitted speed is visible to the driver.

What the other sidewall markings mean

The size is only part of what is moulded into the rubber. Several other markings change what the tyre is suitable for.

• XL, RF or Reinforced. Extra load construction, able to carry more weight at higher pressure. If the car came with XL, replace with XL.
• M+S. Mud and snow. Self declared by the manufacturer with no test behind it, so it means very little on its own.
• Three peak mountain snowflake. Awarded only after a snow traction test. This is the marking that identifies a genuine winter or all season tyre.
• Run flat. Marked variously as RFT, ROF, ZP, DSST, SSR or RSC depending on the maker. Reinforced sidewalls let the tyre be driven on for a limited distance after a puncture. Only fit run flats to a car designed for them, and never mix run flats with conventional tyres.
• DOT code. A four digit number giving the week and year of manufacture. 2521 is the twenty fifth week of 2021.
• Original equipment codes. A star for BMW, MO for Mercedes, AO or RO1 for Audi, N0 to N6 for Porsche, VOL for Volvo, and so on. These indicate a version of the tyre developed with that manufacturer, and on some cars the difference in behaviour is noticeable.
• Rotation arrow or Outside marking. Directional and asymmetric tyres must be fitted the right way round or they lose a good deal of their wet performance.
• Tread wear indicators. Small raised bars in the main grooves, level with 1.6mm. If the tread is flush with them, the tyre is at the legal limit and should already have been replaced.

The tyre label displayed at the point of sale adds three more pieces of information: a fuel efficiency grade, a wet grip grade and an external noise figure in decibels, plus icons for snow and ice performance where they apply. Wet grip is the one worth paying attention to, since the difference between the top and bottom grades shows up as several car lengths in an emergency stop.

How to find the correct tyre size for your car

Use the manufacturer placard rather than the tyres currently fitted, because the tyres on the car may be wrong.

• The placard. A sticker in the driver's door shut, inside the fuel filler flap or occasionally in the glovebox lid. It lists the approved sizes and the correct cold inflation pressures for normal and fully laden loads.
• The handbook. Lists the same sizes, often with more than one approved fitment.
• The current tyres. Read the sidewall, but treat it as a cross check. Check all four match each other and match the placard.
• A tyre retailer. Registration lookup tools are convenient but not infallible, particularly where a model had several wheel options. If the lookup disagrees with your placard, trust the placard.

Your V5C registration document does not list tyre sizes, so there is no point looking there.

Why the correct tyre size matters

The wrong size changes how the car brakes, steers and reads its own speed, and it can invalidate your insurance.

Handling and braking. Suspension geometry, ABS and stability control are all calibrated around a specific rolling circumference and contact patch. Change it and the systems are working from wrong assumptions, which shows up when you need them most.

Speedometer accuracy. A tyre with a larger rolling circumference travels further per revolution, so the speedometer under reads and the odometer under counts. UK law requires a speedometer never to under read, so this is not a trivial point. Any size change should keep rolling circumference within roughly 3 percent of standard.

Clearance. A tyre that fouls the arch liner, suspension components or brake caliper on full lock or full compression will destroy itself, and possibly a wing liner, without much warning.

Fuel and wear. The wrong size or the wrong pressure for that size produces uneven wear, higher rolling resistance and shorter tyre life.

Legal position. UK regulations require at least 1.6mm of tread across the central three quarters of the tread and around the whole circumference, with no cuts, lumps or bulges. It is also an offence to mix radial and cross ply tyres on the same axle. Penalties apply per tyre, so a full set of illegal tyres can put a licence at risk.

Plus sizing: bigger wheels, lower profile tyres

Plus sizing means fitting a larger diameter wheel with a lower profile tyre so that the overall rolling circumference stays about the same.

Done properly, going up an inch in rim diameter and down in aspect ratio keeps the speedometer honest while giving a wider contact patch and a stiffer sidewall.

What you gain:

• Sharper turn in and less sidewall flex under cornering
• More stable feel at speed
• The look, which is usually the real reason

What you give up:

• Ride comfort, because there is less rubber to absorb bumps
• Pothole resistance. Low profile tyres and large rims are far more likely to be damaged by a UK pothole, and the wheel takes the impact rather than the tyre
• Kerb clearance. A larger rim with a thin sidewall sits closer to the kerb face, so kerbing damage becomes much more likely
• Cost, since larger sizes are more expensive to replace
• Sometimes fuel consumption and refinement

Two practical points. Notify your insurer if you fit non standard wheels, because undeclared modifications are a common reason for claims being reduced or refused. And accept that low profile tyres and big rims make wheel damage a question of when rather than if. Kerbed rims and pothole marks are one of the most common jobs we see, and both are usually repairable. Standard painted or powder coated wheels can be refinished through alloy wheel refurbishment, while machined faces need diamond cut wheel repair, which recuts the face on a lathe before relacquering.

Common mistakes when buying tyres

Most tyre buying mistakes come from focusing on price and size while ignoring the two coded figures at the end.

• Dropping the load index or speed rating to save money. Both are part of the vehicle's type approval.
• Buying two tyres and fitting them to the wrong axle. New tyres should generally go on the rear, even on a front wheel drive car, because a car that loses grip at the back first is much harder to recover.
• Mixing tread patterns across an axle. Two different patterns on the same axle behave differently under braking and in the wet.
• Ignoring the date code on a bargain set. A cheap tyre manufactured six years ago is not a bargain.
• Fitting a size the seller says is close enough. A 10mm difference in width or a step in profile changes rolling circumference and can foul on full lock.
• Forgetting the spare. If your car has a full size spare, it needs the same care, correct pressure and a date code check.
• Assuming a registration lookup is definitive when a model was sold with several wheel options.

When to check your tyres

Check pressures and condition at least monthly, and always before a long journey.

Set pressures cold, using the figure from the placard rather than the maximum moulded into the sidewall, and use the laden figure if the car is loaded up for a holiday. Look at tread depth at the inner, middle and outer edges rather than just the middle, because uneven wear tells you something. Wear on both edges usually means under inflation, wear in the centre means over inflation, and wear on one edge alone points to alignment.

Also check the size, construction and pattern actually match across each axle, particularly on a used car where nobody knows what happened before you owned it. It takes thirty seconds and it is the sort of thing that only becomes obvious when the road is wet and you need the car to stop.