When two tyres are worn but two still have life, the sensible unit of replacement is the pair, and which axle the new pair goes on matters more than most drivers expect.
Why pairs
Tyres are replaced across an axle, both fronts or both rears, so the grip is even from side to side. That balance is what keeps braking straight and cornering predictable. A substantial mismatch across an axle can affect braking and handling, which is why a matched pair is the default when two need doing.
The pair should match in make, model and size, so the two sides behave identically. Mixing different tyres across an axle reintroduces exactly the imbalance pairing is meant to avoid.
When a single tyre is fine
A pair is not always necessary. Replacing one tyre is reasonable when:
- The other three are the same type, and
- The remaining tyres meet the vehicle manufacturer's matching and circumference requirements
This is the common case after a single puncture on a fairly new set, where one fresh tyre sits happily among three barely-worn ones. It is only when the others are well down that a lone new tyre creates a meaningful mismatch.
New ones on the rear
The rule that surprises people: a new pair goes on the rear axle, not the front, even on a front-wheel-drive car. The reasoning is stability. Deeper tread at the back means the rear grips when the front is at its limit, so in a wet bend or an emergency stop the car tends to run wide gently rather than the back stepping out into a spin, which is far harder to catch. This is the settled advice of tyre-safety bodies and tyre makers alike. A good fitter moves the better tyres to the back as a matter of course.
Putting it together
So: replace in matched pairs across an axle, keep a single new tyre only among others of similar tread, and put the freshest rubber on the rear. Fitting a properly matched pair is straightforward to arrange: ordered together from an online tyre shop like Tyres.co.uk and booked in at a local fitter, then set on with the deeper tread to the back. For cars where the question is really about all four, replacing the full set covers the AWD case.
There is no universal tread-difference allowance
Do not use a generic two- or three-millimetre difference as permission to fit one new tyre. The acceptable difference depends on the vehicle, drivetrain and tyre requirements. Check the manufacturer's instructions and have the remaining tyres measured. A nearly new matching tyre is a stronger candidate for a single replacement than one with substantial wear or a different construction.
Why the axle decision matters
Recommendations to put the deeper tread on the rear are intended to support stability when grip is reduced, especially in the wet. They do not override different front and rear sizes, directional fitment requirements or vehicle-specific instructions. Ask the fitter how the advice applies to your car rather than moving tyres between axles yourself.
What to authorise
Ask which tyre is beyond use, whether it is repairable if punctured, and why the proposed number of replacements is appropriate. Confirm the pattern and specification of the replacements and the positions in which they will be fitted. If an all-wheel-drive system has tight matching tolerances, the manufacturer's requirements should guide the decision rather than a general sales rule.
Common questions
Should tyres be replaced in pairs?+
Ideally yes, across an axle. Fitting two matched tyres to the same axle keeps grip even from side to side, which matters most in braking and cornering. A single new tyre alongside a worn one creates an imbalance, so a pair is the safer default.
Is it OK to replace just one tyre?+
Yes, if the other three are the same type and within roughly 2 to 3mm of tread. That is common after a puncture on a newish set. If the others are noticeably more worn, the mismatch across the axle makes a pair the better choice.
Should new tyres go on the front or rear?+
The rear. A new pair with deeper tread goes on the back axle, because more grip at the rear keeps the car stable in the wet and helps avoid a sudden loss of control. This applies even to front-wheel-drive cars, where many drivers expect the opposite.
Why not put the new tyres on the front?+
Because rear grip is what keeps a car stable. If the rear lets go before the front in the wet, the car can spin rather than simply run wide, which is far harder to correct. Keeping the deeper tread at the back protects against that, whichever wheels drive the car.