Written by the fitting team at 24/7 Mobile Tyre Services, Glasgow. We fit tyres for a living, and we still wrote this. Last reviewed: July 2026.
You went in about one tyre. You are now standing at a counter in Shawlands or Motherwell holding a quote for four, and the number at the bottom is a lot bigger than the one in your head.
Here is the honest headline. Sometimes four is correct. Sometimes two is correct. Occasionally one is correct. And you can work out which in about five minutes without knowing anything about cars.
This is not an article telling you that garages are crooks. Most are not, and believing that would leave you driving on something dangerous out of stubbornness. What it will do is hand you three questions that sort a genuine recommendation from a soft one.
Quick answer: Legally, you only need to replace tyres that fall below 1.6mm of tread or are damaged. Four new tyres are genuinely necessary in specific cases, most commonly on all wheel drive vehicles, where mismatched rolling circumference can damage the drivetrain. Ask the garage whether the recommendation is a legal requirement, a manufacturer requirement, or advice, and ask to see the tread readings.
Start with the three questions
Say these out loud. Word for word if you like. A good garage will answer all three without blinking.
Question 1: “Is this a legal requirement, a manufacturer requirement, or your advice?”
Three completely different answers, and only two of them are non negotiable.
Legal means the tyre is below 1.6mm across the central three quarters of its breadth, or it is damaged, or the combination on that axle is prohibited. That is not a discussion. It has to be fixed.
Manufacturer means your specific vehicle requires it. All wheel drive circumference rules, run flat construction, staggered fitments. Also not really a discussion, if it applies to your car.
Advice means something like “you are down to 3mm and you will be back in six months.” That is legitimate, useful professional judgement. It is also entirely your call.
A weak recommendation usually falls apart at this question. A strong one gets sharper.
Question 2: “Can you show me the readings, inner, centre and outer, on each tyre?”
This is the one that does the real work.
A tyre can read 5mm on both shoulders and 1.4mm in the middle, which is classic over inflation wear. Or the reverse, thick in the middle and worn at the edges, which is under inflation.
A single centre reading proves nothing either way. Three points per tyre, written down, is the professional standard.
Here is the reassuring part. A garage that measures properly is usually pleased to show you. Asking is not a confrontation, and a decent fitter will read it as a customer who is paying attention.
Question 3: “What caused the wear, and has that been fixed?”
Uneven wear is a symptom, not a fault in itself.
- Feathering across the tread blocks points at toe alignment
- Inner edge wear points at camber or worn bushes
- One sided wear often follows a pothole strike or a heavy kerb hit
Buy four tyres without correcting the cause and you are buying four tyres twice. That is a far more expensive mistake than the quote in your hand.
If the car has been pulling since its last set, our guide on why a car pulls to one side after new tyres covers what to check.
When four really is the right answer
This section matters just as much as the last one, because refusing four when you genuinely need four is its own kind of expensive.
All wheel drive and 4×4. This is the big one.
AWD systems are engineered around four tyres of closely matched rolling circumference. When one tyre is meaningfully smaller than the others, its wheel spins slightly faster. The system reads that as slip.
Depending on the setup, that puts continuous load through the centre differential, viscous coupling or Haldex unit. It is not a theoretical risk, and drivetrain repairs cost several times what a set of tyres does.
Most manufacturers publish a maximum permitted variation, and the tolerances are small. They also differ between vehicles, so check your handbook, both the tyres section and the transmission section. Do not rely on a number you read on a forum.
The ones we meet most often around Glasgow: Audi Quattro, BMW xDrive, Subaru, Volvo XC models, Land Rover, Nissan X-Trail, Ford Kuga and VW Tiguan.
All four are genuinely at or near the limit. Sometimes the boring answer is the true one. If every tyre reads under 2mm, four is four.
Damage on more than one corner. A pothole or a kerb strike frequently takes two at once. If the other two are already marginal, doing the lot makes sense.
Age. Sidewall cracking and old DOT codes across the set is its own conversation, and we covered it properly in old tyres with good tread.
Run flats. Mixing run flat and standard construction on a vehicle designed around run flats is a manufacturer issue, not a preference.
Staggered fitments where the front and rear sizes differ by design, common on performance cars.
When two is the right answer, and where they must go
For most front and rear wheel drive cars, the correct default is simple: replace in axle pairs. Both tyres on the same axle matched in brand, pattern and tread depth.
Now the part most drivers get wrong.
New tyres go on the REAR axle. Yes, even on a front wheel drive car.
The reasoning is about how a car loses grip rather than how it drives. Deeper tread at the rear preserves wet weather grip where you need it most, because when the rear lets go you get oversteer, and oversteer is far harder for an ordinary driver to catch than front end understeer.
This is standard guidance from the major tyre manufacturers, not a workshop opinion.
Exceptions, honestly stated: staggered sizes, vehicles specified with directional tyres at one end and asymmetric at the other, and any case where the manufacturer says otherwise. The handbook always wins.
One practical consequence worth knowing before you agree. Moving the good rears to the front means four wheels come off, not two, TPMS sensor positions change, and pressures front to rear are usually different. That is normal, it just takes longer than people expect.
What the law actually says, and what it does not
| Legal on the same axle | Illegal on the same axle |
|---|---|
| Different brands | Mixing radial and cross ply construction |
| Different tread patterns | Cross ply or bias belted at the rear with radial at the front |
| Different tread depths | Any tyre below 1.6mm in the central three quarters |
The minimum is 1.6mm across the central three quarters of the breadth, around the entire circumference. Below that is illegal, and the penalty is up to £2,500 and 3 penalty points per tyre.
Now the important nuance. The law sets a floor, not a target.
Wet stopping distances start lengthening well before you reach 1.6mm. That is why 3mm is a widely recommended change point across the industry.
To be completely clear: 3mm is advice, not law. Anyone telling you a 2.5mm tyre is illegal has got it wrong. It is legal, and it is also not doing you many favours in a Glasgow downpour.
More on legal combinations in our guide to different tyres on the same axle.
Part worn tyres: the cheaper option that usually is not
Part worns are legal to sell in the UK, but they are tightly regulated. They must be marked “PART WORN”, must meet minimum requirements including a minimum tread depth of 2mm, and must be free of specified damage.
The practical problem is not the regulation. It is what you cannot see.
You cannot inspect internal structural damage. You cannot tell whether the tyre had a bad repair in a previous life. You cannot know how it was stored, or for how long.
Then do the arithmetic. A part worn arriving at 2mm has already used roughly three quarters of its usable life. A new tyre starts around 8mm. Two part worns at 2mm rarely beat one new tyre at 8mm on cost per mile, and they definitely lose on wet grip.
Getting a second opinion without losing your car for a day
Here is the awkward reality. By the time you get the quote, the car is often already on the ramp with the wheels off.
You are still entitled to pause. A polite, workable line: “I’d like to think about it. Can you refit and I’ll come back to you.” Any reasonable garage will do that.
The alternative that does not cost you a day is having someone measure the tyres wherever the car already is. At home, in the work car park, on the driveway.
We do this regularly. Gauge all four at three points, photograph the readings, hand you the numbers, and quote. If the answer is two, we fit two and say so. If the answer is four, we will tell you that as well, and explain why.
That is not us being generous. A customer who trusts the answer comes back.
The checklist before you say yes
- Photograph the quote and each tyre’s sidewall
- Get the tread readings in writing, three points per tyre
- Ask specifically which tyres are below the legal limit
- Check the size, load index and speed rating on the quote match the sidewall or the door pillar plate
- Ask whether wheel balancing, new valves and disposal are included
- Ask what caused the wear and whether alignment is needed
- If AWD, ask what your manufacturer’s permitted variation is
- Confirm the new tyres are going on the correct axle
A job we are quite proud of
A customer rang us from a car park in the Southside. She had been quoted for four and wanted a second opinion before she paid.
We came out, gauged all four at three points, and photographed every reading for her. Two were genuinely finished. The other two were sitting comfortably above 4mm with even wear across the tread.
We sold half as many tyres that day as we could have. She has since sent us three neighbours. That trade works out fine.
FAQs
Is it illegal to have different tyres on the front and back? No. Different brands, patterns and tread depths across axles are legal. What is prohibited is mixing radial and cross ply construction on the same axle, and running cross ply at the rear with radial at the front.
Can I replace just one tyre? Legally yes, on most two wheel drive cars, as long as the other tyre on that axle is in similar condition. Best practice is to replace in pairs so grip is balanced across the axle. On AWD vehicles, a single tyre is often a problem.
Do new tyres go on the front or the back? The back, regardless of which axle drives the car. Deeper tread at the rear protects against oversteer and aquaplaning. Exceptions apply for staggered fitments and where the manufacturer specifies otherwise.
Do I have to replace all four tyres on a 4×4? Often, yes. AWD systems need closely matched rolling circumference and the tolerances are small. Check your handbook for your vehicle’s permitted variation rather than relying on a general figure.
What tread depth should I replace tyres at? The legal minimum is 1.6mm. Most of the industry recommends replacing at around 3mm, because wet braking performance drops noticeably below that. The 3mm figure is guidance, not law.
Are part worn tyres a false economy? Usually. They are legal when correctly marked and sold to the minimum standards, but you cannot see internal damage or previous repairs, and starting at 2mm leaves very little life.
Can I refuse the tyres the garage recommends? Yes, unless a tyre is illegal or dangerous. Advisories are advice. Ask for the readings in writing, then decide.
Will different brands on the same axle fail the MOT? Not by itself. Different brands and patterns on the same axle are legal. Mixed construction types are not, and any tyre below 1.6mm will fail regardless of brand.
Want the tyres measured properly before you commit?
We will come to the car. Home, work, or wherever it is parked. We gauge all four at three points, show you the readings, and tell you honestly whether you need one, two or four.
If the answer is two, we fit two.
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Call 07955 533000 or message us on WhatsApp. Or get a quote here.
Related reading: what to do when your car fails the MOT on tyres, mobile tyre fitting in Glasgow, emergency tyre replacement, puncture repair to BS AU 159, TPMS sensor service.
