Can you replace just one tyre on a 4x4?

Sometimes, but only if the other three are barely worn. On a four-wheel-drive car all four tyres are mechanically linked, so a new tyre alongside three worn ones makes one wheel turn at a slightly different speed all the time. Most handbooks allow a difference of only 2–3 mm of tread across the car before that starts to damage the centre differential or coupling.

Do you have to replace all four tyres on an all-wheel-drive car?

Not always. If the remaining tyres still have close to full tread you can usually fit a single or a pair. Once the others are halfway worn, four is the safe answer — and on some cars, notably Subarus, the handbook says four regardless.

What happens if you put mismatched tyres on a 4x4?

The drivetrain has to absorb the speed difference between the axles. On a viscous coupling or Haldex-type system that means constant slip, heat and wear; on a car with a locked or limited-slip centre differential it means wind-up. The repair bills run from several hundred pounds to a few thousand, against the price of one tyre.

Guide

Replacing tyres on a 4x4: one, two or all four?

One tyre is ruined and the other three are fine. On a front-wheel-drive car that’s a simple decision. On a four-wheel-drive one it isn’t — and getting it wrong is expensive in a way that doesn’t show up for months.

The short answer: if the other three tyres are nearly new, one is usually fine. If they are halfway worn, you are looking at four. The dividing line is roughly 2–3 mm of tread difference across the car — and your handbook has the number that actually applies to yours.

Why a 4x4 is different

On a two-wheel-drive car, the two driven wheels are joined by a differential whose whole job is to let them turn at different speeds. A new tyre next to a worn one is no drama.

On a four-wheel-drive car, the front and rear axles are joined together as well — through a centre differential, a viscous coupling, a Haldex-type clutch pack or an electronically controlled one, depending on the car. That connection is designed to soak up the small, short-lived speed differences that happen when you corner or when a wheel slips on a wet drain cover.

A tyre with a different rolling circumference creates a speed difference that never goes away. Every mile, at every speed, in a straight line on dry tarmac, that coupling is being asked to absorb a mismatch it was never meant to see continuously.

What that actually costs you

The damage is gradual, which is what makes it such an easy mistake to make — the car drives perfectly well for months.

Viscous couplings cook. The silicone fluid inside degrades with heat, the coupling seizes or stops working, and you end up with a car that is quietly front-wheel-drive, or one that binds in tight turns.
Haldex and clutch-pack systems wear their friction plates out prematurely, and the pump and controller work harder than they should.
Centre differentials in permanent four-wheel-drive systems wind up torque they can’t release, which loads the bearings and gears constantly.
Transfer cases on part-time systems suffer the same way whenever four-wheel-drive is engaged.

None of that is a cheap fix. A coupling or Haldex unit is typically several hundred pounds fitted, a transfer case or centre diff comfortably into four figures. And it won’t be a warranty claim — mismatched tyres are exactly the sort of thing that gets a claim refused.

There is a second, more immediate problem: your car’s ABS, traction control and stability systems all work by comparing how fast each wheel is turning. Feed them a permanent mismatch and you can end up chasing warning lights that have nothing wrong behind them except the tyres.

So how close is close enough?

The honest answer is that it depends on your car, and the handbook is the authority. But the rules of thumb the trade works to:

Within 2–3 mm of tread depth across all four is the figure most manufacturers land on.
Subaru is the strictest of the mainstream makes — their symmetrical system has very little tolerance, and their guidance amounts to replacing all four.
Performance and premium AWD — Audi quattro, BMW xDrive, Mercedes 4MATIC, Porsche — generally want four, or at the very least a matched axle pair of the same make and model of tyre.

To put the numbers in perspective: a new tyre has around 8 mm of tread and the legal minimum is 1.6 mm. A 5 mm difference between a new tyre and a worn one changes the overall diameter by about 10 mm, which on a typical 4x4 size is roughly a 1.5% mismatch — permanently. That is more than enough to matter.

You can work out exactly how far apart two sizes are with our rolling radius calculator. It’s built for comparing different sizes rather than different wear, but the arithmetic is the same one your drivetrain is doing.

Your options, in order of preference

1. Four tyres

Always correct, never argued with, and the only option if the other three are meaningfully worn. It’s the expensive answer up front and the cheap one over the life of the car.

2. A pair across one axle

The minimum you should ever consider. Two new tyres go on the same axle — never one front and one rear — and they should be the same make and model as each other. This is reasonable when the other axle still has plenty of tread and you’re inside the handbook’s tolerance.

If the two new ones go on the rear, be aware that on a car with a lot of tread difference front to rear the handling balance changes.

3. A single tyre

Only when the other three are close to new — realistically, within a couple of millimetres. That’s the situation where a single is genuinely fine: a nail through a nearly-new tyre in a spot that can’t be repaired. Match the make and model to the others if you possibly can, because two tyres of the same nominal size from different manufacturers are not always the same height.

4. Shaving a new tyre to match

A specialist can machine a new tyre down to the tread depth of your existing ones. It exists, it works, and it’s common on cars where four tyres is a serious bill. It’s a niche service in the UK, though, and by the time you’ve paid for a tyre plus the shaving, the sums often don’t work out. Worth knowing about; rarely worth doing.

First, check whether it can just be repaired

All of this assumes the tyre needs replacing. A puncture in the central three-quarters of the tread, under 6 mm, in a tyre that hasn’t been run flat, can usually be properly repaired — and a repaired tyre keeps its original diameter, which sidesteps the whole problem. Our guide on whether a puncture can be repaired covers where the line is.

This is the single most useful thing to establish before spending money on a 4x4, because the difference between a repair and a replacement can be the difference between £35 and four new tyres.

A word on the spare

If your 4x4 has a full-size spare, the same logic applies to it: a spare that’s been sat under the boot floor for six years with 8 mm of tread is mismatched against four half-worn tyres. Fine to limp home on, not fine to leave on for a fortnight.

If it has a space-saver, check the handbook before driving any distance — on a lot of all-wheel-drive cars the guidance is to keep the mileage and speed right down, precisely because of the mismatch.

What we do about it

When we get called to a 4x4 with one destroyed tyre, the first thing we do is measure the other three. That tells us whether this is a one-tyre job, an axle pair or a set of four — and it’s a conversation worth having before we order anything rather than after.

We fit at your home, your work or the roadside, and we can bring a set of four just as easily as one. See mobile tyre fitting for how it works, or ring us with your registration and we’ll tell you what your car takes.