Tyres do not only wear out, they age out. The compounds that keep rubber flexible break down over time under sunlight, heat and ozone, and the visible result is cracking: fine lines at the base of the tread grooves, or a crazed pattern around the sidewall. It is common on low-mileage cars, caravans and spare wheels, where tread depth is no guide at all.
Where cracking shows first
Look in three places:
- The base of the tread grooves, where the rubber flexes as the blocks deform.
- The sidewall flex zone, roughly the middle band of the sidewall, which bends on every revolution.
- Around the bead area, particularly on tyres that have sat unused.
Cracks tend to run circumferentially, following the direction of flex. A tyre can look fine head-on and show clear crazing when you squat down and look along the sidewall in good light.
Telling cosmetic from serious
The distinction is depth and location:
- Fine surface crazing, shallow lines you can see but barely feel, on an otherwise sound tyre: monitor it. This is normal ageing.
- Deep cracks you can get a fingernail into, or that open when the sidewall is flexed by hand: replace.
- Any crack showing cords or fabric: replace immediately. The casing is compromised.
- Cracking plus a bulge: replace immediately, because a bulge means the casing has failed.
Perished rubber also feels different: harder and less elastic than a healthy tyre, and it may look dull and grey rather than black.
Why it happens
Three things drive it. UV light breaks down polymers at the surface. Ozone in the air attacks stretched rubber, which is why flex zones crack first. And heat accelerates both.
Anti-ageing compounds in the rubber migrate to the surface to protect it, and that process depends partly on the tyre being flexed. This is the counter-intuitive part: a tyre that is used regularly often ages better than one parked for years, which is why low-mileage cars and stored caravans suffer most. It is also why the date code is worth reading, and our tyre date code checker works out the age from it. Heat from an under-inflated tyre flexing too much, or simply from a summer of parking on hot tarmac, speeds the process up.
The legal position
UK law does not set a maximum age for tyres on cars, but it does require them to be free from certain defects. Cuts, lumps, bulges and any condition where the ply or cord is exposed make a tyre unroadworthy, and that applies whatever the tread depth. The requirements are set out on GOV.UK.
So a badly perished tyre with 6mm of tread can still be illegal, and it can still fail an MOT, which we cover in why tyres fail the MOT.
Reading the date code
Every tyre carries a four-digit code that gives the week and year it was made, and on an older tyre that number matters more than the tread depth. Find the letters DOT on the sidewall and follow the string of characters to the end.
The last four digits are the date: the first two are the week, the second two the year. So "3221" is the 32nd week of 2021. The code is often only moulded on one side, so you may need to look at the inner sidewall.
There is no legal maximum age for tyres on a car, but tyre makers generally advise a professional inspection from around five years and replacement by ten regardless of appearance. For lorries, buses and coaches, tyres over ten years old are banned from the front axle, which tells you how seriously the regulator treats age on vehicles that carry people.
Our guide to how old is too old for a tyre goes into the reasoning.
Two practical points. A tyre that has sat in a warehouse for two years before it was fitted has already used two years of its life, so check the code on new tyres before they go on. And a three-digit code means the tyre was made before 2000, which makes it too old for any road use.
Caravans, spares and stored cars
The tyres most likely to be badly perished are the ones that get looked at least: the spare in the boot, the caravan on the drive, and the second car that only comes out at weekends. Each has its own trap.
- The spare wheel is protected from sunlight but still ages, loses pressure and can crack around the bead. Check its date code and pressure at least yearly. A space-saver is often the original tyre from when the car was new.
- Caravans and motorhomes stand loaded for months in one position, so the sidewall flex zone is stretched over the same section all winter. Cracking here is the classic cause of a caravan blowout on the first trip of the season, see caravan and trailer tyre safety.
- Low-mileage and stored cars can have 6mm of tread on tyres old enough to be dangerous. Age, not wear, sets the replacement date.
- Trailers and horseboxes often run the tyres they were sold with a decade ago, and the loads are heavy.
For any vehicle that stands idle, moving it a short distance every few weeks and keeping the tyres inflated to the top of the recommended range slows both cracking and flat spots.
Cracked tyres and the MOT
The MOT does not fail a tyre for age or for fine surface crazing, but it does fail cracks and cuts that reach the structure. This is roughly how the tester will see it:
| What the tester finds | Likely outcome |
|---|---|
| Fine surface crazing, rubber otherwise sound | Pass, possibly with an advisory noting the age or condition |
| Deeper cracking in the sidewall or tread grooves, cords not visible | Advisory or failure depending on depth, at the tester's judgement |
| Any cut or crack deep enough to reach the ply or cord, if it is longer than 25mm or 10% of the tyre's section width, whichever is greater | Failure |
| Cord or fabric visible anywhere | Failure |
| A lump or bulge caused by separation of the structure | Failure |
An advisory is not a pass on the underlying problem. A tyre that gets an age or cracking advisory in spring will be worse by the following winter, and the tester's job is to apply the standard on the day, not to predict the next twelve months. Treat an advisory for perishing as a replacement to plan for now.
The legal requirements are on GOV.UK.
Replace one, or all four?
If one tyre is perished, the others of the same age almost always are too, so check all five before you decide. Cracking is driven by time, light and heat, and all four tyres on a car have usually had the same of each.
Where the cracking is genuinely confined to one tyre, for example the sunny side of a car that always parks the same way round, replace that tyre and its partner across the axle so that grip and tread depth match.
Where all four are the same age and all show cracking, replacing all four is the honest answer, and it removes the risk of a second failure a month later.
Fit the new pair to the rear if you are only doing two. Rear grip is what keeps a car stable in the wet, and most fitters will recommend it. Whichever you choose, ask for the date codes on the new tyres so you are not paying for stock that has already spent years on a shelf.
How to slow it down
You cannot stop ageing, but you can avoid accelerating it: keep tyres correctly inflated so the sidewall flexes as designed, park out of direct sunlight where you can, keep petrol, oil and solvents off the rubber, and avoid silicone-based tyre shine products that can dry the surface. For a stored set, follow the conditions in seasonal tyre storage.
If you are not sure whether what you are looking at is normal crazing or a problem, send a clear photo of the sidewall on WhatsApp to 07473 720871 and we will give you our honest view on whether it needs replacing. If the tyres do need replacing, we fit new ones through our mobile tyre fitting service at your home or work.
