"Do I really need all four?" is one of the most common questions at the tyre counter, and the honest answer is: sometimes yes, often no. It depends mostly on what drives the wheels.
Two-wheel-drive: a pair is usually fine
On a normal front- or rear-wheel-drive car, there is no need to replace all four if only two are worn. The standard practice is a matched pair across an axle, both fronts or both rears together, so the grip is balanced side to side. Replacing in pairs keeps the car predictable without spending on tyres that still have life in them.
All-wheel-drive: often all four
All-wheel-drive and four-wheel-drive cars are the exception. Their systems split power between the axles and assume all four tyres are turning at the same rate. Different tread depths mean slightly different rolling diameters, which can make the system fight itself and, over time, strain the differential or transmission.
Because of that, many AWD makers require all four to be matched within a tight tread tolerance, and replacing all four together is often the safe answer, sometimes the required one. A few allow a single new tyre to be shaved to match the others. The handbook is the authority here, and the cost of ignoring it can be a far bigger repair than a set of tyres.
The new pair goes on the rear
Whichever wheels drive the car, there is one rule that catches people out: a new pair belongs on the rear axle. Deeper tread at the back keeps the car stable in the wet, because if grip is going to let go, understeer at the front is far more manageable than the rear stepping out. This holds even on front-wheel-drive cars, where instinct says to put the new ones on the driven front wheels. A good fitter will move the better tyres to the back.
Keep them matched
Whatever the count, the tyres on an axle should match, same make, model and size, so grip is even across it. Mixing different types or brands on one axle upsets the balance and is best avoided.
So the practical answer is usually a matched pair, or all four on an AWD car, with the freshest rubber on the rear. Buying a full matched set online is simple: order the right size from a tyre site such as Tyres.co.uk, and the fitting can be booked at a local centre in one go, so all four go on together and properly matched. The follow-up question of pairs versus singles is worth reading next.
From the workshop: the all-wheel-drive one genuinely matters, I've seen people fit one odd tyre to a Subaru or a Quattro and chew up a centre diff that costs more than four tyres. On a normal hatchback, though, a matched pair on the back is fine. Don't let anyone sell you four when two will do, and don't skimp on an AWD when it needs four.
Sources and accuracy. This reflects general drivetrain and safety guidance at the time of writing; AWD tolerances vary by car and the handbook is definitive. If anything here looks wrong, get in touch and we will check it and put it right.
Common questions
Do you have to replace all four tyres at once?+
Not usually on a two-wheel-drive car, where replacing a matched pair across an axle is standard. On all-wheel-drive cars it is often necessary, because mismatched tread can strain the drivetrain. The handbook is the authority for a given car.
Why do AWD cars need all four tyres replaced together?+
All-wheel-drive systems share power between axles and expect all four tyres to turn at the same rate. Different tread depths mean slightly different rolling diameters, which can make the system work against itself and, over time, damage the differential or transmission.
Can I just replace one tyre?+
Sometimes, if the other three have similar tread and are the same type, common after a single puncture on a fairly new set. If the others are well worn, a single new tyre creates a grip mismatch, so a pair, or on AWD all four, is the safer answer.
Where should new tyres go, front or rear?+
On the rear. A new pair with deeper tread is fitted to the back axle for stability, because more rear grip helps the car stay predictable in the wet. This holds true even on front-wheel-drive cars, which surprises many drivers.