How to read tyre size

What does 225/45 R17 94Y actually mean?

The mixed units — metric width, imperial rim — are a historical quirk of how the sizing convention evolved, and they are here to stay. What matters is that the pattern never changes: width, profile, construction, rim, load, speed, in that order, on every car tyre sold in the UK.

225/45 R17 is among the most common fitments on UK family and compact cars — there are currently 4 tyres in that size in the LCDrive range, and you can browse every 225/45 R17 tyre to see the spread of brands and prices. The same six-value logic decodes any size you meet.

What is the load index and why does it matter?

The load index is the maximum weight one tyre can carry at its rated pressure, expressed as a code number. It is not a weight in itself — 94 does not mean 94 kg — so you need the reference table:

Multiply by four for the whole car: a set of 94-rated tyres can support 2,680 kg between them. The rule that matters is simple — never fit a tyre with a lower load index than the vehicle manufacturer specifies. A lower-rated tyre run at full vehicle load flexes more and runs hotter, and the shortfall may only reveal itself on a fully laden motorway run. Going higher than specified is fine; going lower is not.

Indices climb with vehicle weight. Superminis sit in the high 80s, family saloons around 91 to 94, and the SUV and van tyres in the range carry indices of 107 and above — which is one reason an SUV tyre is not just a bigger car tyre.

What does the speed rating letter mean?

The speed rating is the maximum speed the tyre can sustain, fully laden, without overheating. Each letter maps to a ceiling:

No UK road permits any of those speeds — the motorway limit is 70 mph — so the rating is not an invitation. It matters because it reflects heat tolerance and construction: a W- or Y-rated tyre is built with a stiffer casing that also shapes how the car steers and rides.

The practical rule mirrors the load index: match or exceed the rating your manufacturer specifies. Fitting a lower rating than specified can matter to your insurer even if you never approach the ceiling. Van and winter tyres are often sold in lower ratings than the summer equivalent — if in doubt, check the handbook before you order.

Where do you find your tyre size?

• The sidewall — the size is moulded into the rubber on the outer face of every tyre currently fitted. This tells you what is on the car right now.
• The tyre placard — a sticker inside the driver's door jamb or fuel filler flap listing the manufacturer's approved sizes and pressures. This tells you what should be on the car.
• The owner's handbook — lists every approved size, including optional larger-rim fitments.

One place the size is not recorded: the V5C. Your log book identifies the vehicle — make, model, engine — but says nothing about tyre sizes, so there is no point looking there.

Check the sidewall against the placard rather than trusting either alone. A previous owner may have fitted a non-standard size, and the front and rear axles can legitimately differ. Or skip the kerbside crouch entirely: look up your tyre size by registration and LCDrive matches the sizes your car left the factory on.

What is a staggered fitment?

A staggered fitment means the rear tyres are a different size from the fronts — usually wider — and it is common on rear-wheel-drive and performance cars. A sports saloon might run 225/40 R19 at the front and 255/35 R19 at the rear: the wider rears put more rubber under the driven wheels.

Three practical consequences. You buy tyres per axle, not per car. You cannot rotate tyres front to rear to even out wear. And when replacing, you must check which axle you are buying for — the two sizes are not interchangeable, even though they share a rim diameter.

What do XL, run-flat and OEM markings mean?

XL (Extra Load) marks a reinforced casing carrying a higher load index than the standard version of the same size. It is increasingly common as cars get heavier — particularly electric cars and SUVs — and if your car specifies XL, the replacement must be XL too.

Run-flat markings vary by brand, but all mean the same thing: self-supporting sidewalls that hold the car's weight after a puncture. The run-flat tyres in the LCDrive range are rated for up to 50 miles of driving after a puncture at reduced speed — you can browse every run-flat tyre in the range to see them together.

OEM markings — MO or MO1 (Mercedes), a star symbol (BMW), AO (Audi), T0 (Tesla) — mark tyres developed with a specific manufacturer and tuned to that marque's preferences. Several tyres in the range carry them. They fit any car the size suits, but a like-for-like replacement keeps the handling balance the manufacturer signed off.

What happens if you fit the wrong tyre size?

A different rolling circumference skews your speedometer and odometer, and can unsettle the calibration behind ABS and stability control. A wider or taller tyre can foul the wheel arch on full lock or under load. And a load or speed rating below specification undermines the safety margins the car was engineered around. If you are weighing up an alternative size, the tyre size calculator shows how far it deviates from your original's rolling circumference before you commit.

Many insurers treat a departure from the manufacturer's approved sizes as a modification that must be declared — an undeclared change risks complications at claim time, so if you change size, tell your insurer. At the MOT, a tyre unsuitable for the vehicle or fouling the bodywork can fail the test, and anything below the 1.6 mm legal tread minimum will — the UK tread depth law guide covers that side in full.

Stick to an approved size in the correct load and speed rating and none of this arises. For quick answers to related questions, the tyre FAQ is the fastest route.