A different tyre size from the one a car left the factory with is possible, but only within clear limits. The size on the door placard is the safe default, and any change has to respect the same rules the original size was chosen to meet.
Keep the overall diameter close
The single most important rule is to keep the overall diameter, the full height of the wheel and tyre, close to the original. Most manufacturers and tyre professionals advise staying within about 3% of the original overall diameter.
This matters because the overall diameter sets how far the wheel travels per turn, which feeds into the speedometer, the odometer and the electronics. Stray too far and the speedometer reading drifts, the ABS, traction control and tyre-pressure systems can be thrown off, and a taller tyre may foul the wheel arch or suspension. A change that swaps wheel size for tyre profile while holding the diameter steady is plus sizing, and is the usual way a larger wheel is fitted safely.
Meet the load and speed rating
A replacement size must still meet or exceed the vehicle's required load index and speed rating. It is fine to go higher; it is not safe or legal to go lower. A tyre that physically fits the wheel but is under-rated for the car's weight is both dangerous and an MOT failure, so the dimensions are never the whole story.
Fit the wheel, match the axle
The new size also has to fit the wheel properly, the rim has to be within the tyre's approved rim-width range, and tyres across an axle must match in size and construction. A new size is normally fitted as a full set, or at least in matching pairs per axle, rather than one corner at a time, and a matching set in the new size can be ordered and booked in for fitting in one go through Tyres.co.uk, which sells and fits tyres.
Check the placard and the insurer
Two checks finish the job. The door placard and handbook list the original size and often one or more approved alternative sizes; staying within those is the simplest safe route. And where a change moves away from the maker's specified sizes, it counts as a modification, so the insurer should be told and the change agreed in advance, otherwise cover can be affected.
From the workshop: most size changes that go wrong are people fixating on the look of a bigger wheel and forgetting the diameter and the load rating. We work from the placard, keep the rolling diameter within tolerance, and check the load and speed rating match before anything goes on the car.
Sources and accuracy. The roughly 3% diameter guidance, the load and speed rules and the insurance point here reflect manufacturer and trade guidance at the time of writing. The definitive sizes for a specific car are on its placard and in the handbook. If anything here looks wrong, get in touch and we will check it and put it right.
Common questions
Can I fit a different size tyre to my car?+
Within limits, yes. A different size must keep the overall diameter close to the original, within about 3% is the usual guidance, meet or exceed the vehicle's load index and speed rating, and physically fit the wheel. The door placard lists the approved sizes.
How much can tyre size change safely?+
Most manufacturers and tyre professionals advise staying within about 3% of the original overall diameter. Beyond that, the speedometer reads further out, and ABS, traction control and tyre-pressure systems can be affected, along with clearance in the wheel arch.
Is changing tyre size legal in the UK?+
It can be, provided the tyre suits the vehicle: an adequate load index and speed rating, the same construction across an axle, and a size that fits safely. Fitting a tyre with too low a load or speed rating, or that fouls the car, is not legal.
Do I need to tell my insurer if I change tyre size?+
If the change is away from the manufacturer's specified sizes, yes. Tyre and wheel changes count as a modification, and insurers advise being told and agreeing the change first, or cover could be affected. Approved alternative sizes in the handbook are not a modification.