Tyres and the MOT: what the tester checks
Tyres are one of the easiest parts of the MOT to check for yourself beforehand, and the rules behind them aren't complicated. Here's what the tester looks at, which faults fail, and why passing the MOT doesn't mean your tyres are fine for the next twelve months.
What the tester looks at
The tester inspects the tyres on the car's wheels on the day. The spare isn't part of the test: if they notice a problem with it, they'll just tell you. On each tyre they check:
- Tread depth, across the tread and all the way round.
- Damage: cuts, lumps, bulges and tears, and any cord showing.
- Matching: tyres on the same axle must be the same size and the same type of construction.
- Fitting: that the tyre is seated properly on the wheel, isn't rubbing on the car, and is fitted the way its sidewall says.
- The valves.
- The tyre pressure monitoring system, on cars first used from 1 January 2012.
- Pressure, as far as whether a tyre is obviously under-inflated.
The tread limit: 1.6mm
For cars, the main grooves of the tread must be at least 1.6mm deep across the central three-quarters of the tread, and around the whole circumference of the tyre. The strips at the outer edges, outside that central band, don't count. Tread below the limit is recorded as a dangerous defect, which is a fail.
Only the main grooves count: the ones with small raised bars, called tread wear indicators, moulded into the bottom. Those bars sit at about 1.6mm, so once the tread has worn level with them, the tyre is at the limit.
The 20p test
- Put a 20p coin into one of the main grooves.
- Look at the outer band of the coin. If you can see any of it, the tread may be close to or below the limit. Get it measured properly.
- Check several places round each tyre and across its width. Tyres don't always wear evenly, and the limit applies all the way round.
The 20p test is a quick check, not a measurement. A tread depth gauge is inexpensive and gives you the actual figure.
What fails, and what doesn't
MOT faults are graded. Dangerous and major faults fail the test. Minor faults pass but are recorded, and advisories are things to keep an eye on. For tyres on a car, the main ones are:
| Fault | Grade |
|---|---|
| Tread below 1.6mm | Dangerous: fail |
| A lump, bulge or tear from the tyre's structure breaking down, or cords exposed or damaged | Dangerous: fail |
| A valve so damaged or misaligned it could cause sudden deflation | Dangerous: fail |
| A cut deep enough to reach the cords | Major: fail |
| Different sizes, or different constructions such as radial and cross-ply, on the same axle | Major: fail |
| A tyre not seated properly on the rim, or rubbing on part of the car | Major: fail |
| A tyre fitted against the instructions on its sidewall | Major: fail |
| Tyre pressure monitoring system not working (cars first used from 1 January 2012) | Major: fail |
| A tyre obviously under-inflated | Minor: pass, recorded |
On cuts, any cut where the cords can be seen fails. A cut longer than 25mm, or 10% of the tyre's width if that's more, also fails if the tester can feel cords at the bottom of it.
A dangerous fault means you shouldn't drive the car until it's repaired. A major fault should be repaired straight away. A common advisory is a tyre worn close to the legal limit: it passes, but it won't be legal for long.
Things people don't expect
- Tyre age isn't checked on cars. The MOT only checks it on vehicles with more than eight passenger seats. That doesn't make an old tyre safe: here's how old is too old.
- Load and speed ratings aren't checked on cars either. They're checked on larger vans and on passenger vehicles with 13 or more seats. Still fit tyres that meet your car maker's specification. Here's how to read the sidewall.
- An asymmetric tyre fitted inside out isn't a fail. The tester will tell you about it, and it's worth putting right.
- A run-flat and an ordinary tyre on the same axle doesn't fail, though it isn't recommended.
The warning light at the test
On cars first used from 1 January 2012, the tester checks the tyre pressure monitoring system. The light normally comes on for a moment when you switch on the ignition, then goes out. If the system has spotted low pressure, the light stays on, and that isn't automatically a fault. A system fault usually shows as the light flashing for a while, then staying on. A system that isn't working is a major fault and fails.
The simple answer is to set all four tyres to your car's figures before the test, then reset the system if your car needs it. Here's how to reset it on your make.
The MOT is a snapshot
A pass means your tyres were legal on the day of the test. The law applies every time you drive. The Highway Code says tyres must have at least 1.6mm of tread across the central three-quarters and all the way round, and must be inflated to the vehicle manufacturer's figures for the load you're carrying. They must also be free of the cuts, bulges and exposed cords that fail an MOT.
The penalty for a defective tyre is a fine of up to £2,500 and 3 penalty points, for each tyre. Four bad tyres could mean 12 points, and 12 points within three years can cost you your licence.
A two-minute check, once a month
- Tread. The 20p test or a gauge, in several places on each tyre.
- Sidewalls. Look for cuts, bulges and anything stuck in the tread, turning the steering to see the front tyres properly.
- Pressures. Checked cold, against your car's figures. You can look them up by registration.
If anything looks wrong before the MOT, sort it before the test. It's the same job either way, and it saves a retest.