Cold Monday morning at Lymm. I'm doing my walkaround before pulling out and I run my hand round the nearside steer tyre like I always do — and my thumb drops into a gash. A proper cut, sidewall, deep enough that I could see something dark in the bottom of it. Cord. Exposed cord.
If I'd missed that and rolled out, and DVSA had pulled me at a checkpoint two junctions up? That's not a chat. That's an immediate PG9, vehicle off the road, and a very awkward phone call to the transport office. All from a tyre I'd have sworn looked fine from the cab.
That's the thing about tyres. They're the most-checked, most-failed item at roadside, and they're also the bit drivers rush through on the walkaround because they're cold, it's dark, and the kettle's calling. So let's go through what actually matters — at your PMI and every single morning.
Why tyres get you done more than almost anything else
DVSA looks at tyres at the roadside and at the annual test. And tyre condition is one of the most common things that triggers a PG9 at the roadside. Why? Because tyres are visible, they're fast to check, and they fail in obvious ways. An examiner doesn't need to crawl under your unit to spot a bulge or a tyre worn past the limit. He can clock it walking past.
Which cuts both ways. If he can see it that easily, so can you. There's no excuse for rolling out on a tyre that was going to fail. The defect was there for you to find — you just had to actually look.
1.6mm — the number you have to know cold
The legal minimum tread depth for goods vehicles over 3,500 kg GVW is 1.6mm. That comes from the Road Vehicles (Construction and Use) Regulations 1986, Regulation 27. And it's not 1.6mm in one lucky spot — it has to be at least 1.6mm across the central three-quarters of the breadth of the tread, and right round the entire outer circumference.
So a tyre that's legal on one side but bald on the inner edge? Not legal. A tyre that's fine for three-quarters of the way round but worn flat on one patch? Not legal. Central three-quarters, whole way round, minimum 1.6mm. That's the test.
What does 1.6mm actually look like? This is where the tread wear indicators earn their keep. TWIs are little raised bars moulded into the bottom of the grooves at exactly 1.6mm depth. When the surrounding tread wears down level with those bars — you're at the limit. Find the TWI markers (usually a little triangle on the sidewall points to where they are) and look in the grooves. Tread sitting proud of the bar, you're good. Tread level with the bar, that tyre's done.
Load index — the numbers on the sidewall, in plain English
Every tyre's got a string of code on the sidewall like 295/80 R22.5 152/148M and most drivers glance at it and move on. Here's the plain-English version.
The first number — 295 — is the section width in millimetres. The 80 is the aspect ratio. R means radial. 22.5 is the rim diameter in inches. Then you get to the bit that matters for legality: 152/148 is the load index, and M is the speed symbol.
The load index tells you how much weight that tyre's rated to carry. Two numbers because it's rated differently for single fitment versus dual fitment. The point you need to take away: the tyre must not be operated beyond its rated load. Put a tyre with too low a load index on an axle that's carrying serious weight and you're outside the law and the tyre's working harder than it was built for.
I'm not going to tell you load index 152 equals X kilos, because you should be reading it off the sidewall and the manufacturer's tables, not off a blog. But know that the rating exists, know it has to match your axle loading, and if a tyre's been swapped on at a roadside fitter at 3am, glance at the index and make sure somebody hasn't fitted a tyre that's underspec for your axle. It happens.
Mixing tyres — what's banned and why your brakes care
You can't mix radial and cross-ply tyres on the same axle. That's a straight ban. The two constructions behave completely differently under load and under braking, and putting one of each on an axle gives you uneven grip and uneven braking across that axle.
Think about what that means at 40 tonnes coming down off the M6 onto J18 Knutsford. You want both wheels on an axle gripping and slowing identically. Mismatched tyres pull, they brake unevenly, and in the wet that's how you end up with a unit that doesn't track straight under heavy braking. Not worth it. Match your tyres on the axle.
While we're on fitment — drive axle typically runs twin tyres, the steer axle usually runs singles of the right load index. The steers are the ones that'll kill you if they let go at speed, so they get the most attention on my walkaround. Always.
The DOT date code — your tyre might be legal and still knackered
Here's one that catches people. A tyre can have loads of tread and still be too old to trust. Rubber perishes. It dries, it cracks, it gets brittle, and an old tyre can fail even with legal tread.
Every tyre's got a DOT code on the sidewall, and the last four digits are the manufacture date — week and year. So 2619 means week 26 of 2019. Manufacturers often recommend replacing tyres over 10 years old regardless of tread depth. So if you're reading 2619 today and it's deep into the 2020s, that tyre's getting on for the end of its sensible life even if the tread's grand.
On a trailer that's been sat in a yard half the year, the spare or the lesser-used axle tyres can be ancient. Check the dates. Cracking round the sidewall — little crazed lines like dried mud — is the visual tell that the rubber's gone off.
What DVSA's actually looking at by the roadside
When you get pulled at a check, the tyre inspection covers the obvious failures:
- Cuts — especially anything deep enough to reach the cord or ply.
- Bulges — a lump in the sidewall means the internal structure's failed. That's a tyre about to let go. Instant PG9.
- Exposed cord or ply — any tyre showing its structure is an immediate fail.
- Tread depth under 1.6mm — measured, not eyeballed, if it's borderline.
- Pressure issues — a visibly underinflated tyre or signs of pressure-related damage.
A cut, a bulge, or a tyre under minimum tread depth gets you an immediate PG9. There's no warning, no "sort it by Friday." The vehicle's prohibited there and then. So the whole game is finding these before you leave, not having DVSA find them for you.
Read the wear pattern — your tyres are telling you about your brakes
This is the bit that separates a driver who checks tyres from a driver who understands them. The way a tyre wears tells you what's wrong elsewhere on the vehicle.
- Worn in the centre, good on the edges? That's overinflation. Too much pressure bulges the centre of the tread out so it carries the load and wears first.
- Worn on both shoulders, good in the middle? That's underinflation. Not enough pressure lets the edges take the load — and underinflation also builds heat, which is how you get blowouts.
- Worn heavily on one side only? That's an alignment problem. The axle geometry's out, or the tracking's wrong, and the tyre's being dragged across the road at a slight angle every mile.
And uneven or unusual wear on a drive or trailer axle can point you at brake or suspension trouble — a binding brake, a tired air bag, worn bushes. If one tyre's wearing weird and its partner isn't, something mechanical is off on that corner. The tyre's the symptom. Flag it, because the cause is going to cost you more than the tyre if you leave it.
My actual tyre walkaround routine
Nothing clever. Just done the same way every time so I can't skip a wheel:
- Start at the nearside steer and work round the whole vehicle in one direction — never zig-zag, that's how you miss one.
- Eyes and hand. Run your hand round the tread and the sidewall where you can reach. Your thumb finds cuts your eyes miss in the dark, which is exactly how I found mine at Lymm.
- Look in the grooves for the TWI bars on anything that looks low.
- Check for bulges by looking along the sidewall, not just at it — a bulge shows up side-on.
- Kick or thump the duals to catch a flat inner you can't see — a dead one sounds wrong.
- Glance at the DOT codes now and then, especially on trailers you don't know.
- Anything that makes you go "hmm" — write it on the defect sheet. A recorded defect is you doing your job. An ignored one is you carrying the can.
Two extra minutes per walkaround. That's all it is. Two minutes against a PG9, a vehicle off the road, and your operator's OCRS score taking a hit because you couldn't be bothered to run your hand round a tyre.
Stay legal on the stuff that's easy to track, too
Tyres you have to check by hand. But your hours? That shouldn't be eating your brain space.
If you're tired of writing tacho infringements into a notebook, ShiftOwt tracks 561/WTD compliance automatically — £5.99/mo for drivers, agency pricing on request. Keep the hours side handled so the only thing you're worrying about on a cold morning at Lymm is the walkaround itself.
That steer tyre cost me a fitter callout and a delayed start. Cheap, in the end. Find them yourself and they're always cheap. Let DVSA find them and they never are.