Tyres are the single most common MOT failure category in the UK. A bulb costs £3 to fix; a tyre costs £70 to £140. That’s partly why so many drivers arrive at a test hoping to scrape through and partly why so many don’t. The rules are not ambiguous, and a tester with a calibrated probe is not making a rough estimate. If you want to avoid the failure, you need to understand exactly what’s being measured — not just the headline 1.6mm figure.
The legal standard in full
The Motor Vehicles (Construction and Use) Regulations 1986 sets the minimum at 1.6mm of tread depth across the central three-quarters of the tyre’s tread band, measured continuously around the full circumference of the tyre. Every word in that sentence does work.
Central three-quarters. The outermost edge strips of the tread — roughly 12.5% each side — are excluded from the legal minimum. The middle 75% of the running surface is what the law is measuring, and what the tester is measuring. A tyre with good tread on the shoulders but worn out in the middle still fails.
Full circumference. The tester doesn’t measure at one point and call it done. They probe the tyre in multiple places around the circumference. A tyre that’s worn at one point — from a specific incident, or from an intermittent brake binding issue — but has 3mm everywhere else still fails at the worn point.
1.6mm minimum. This is the floor, not a buffer. Tyres are the only thing your car has to stop on. Most tyre manufacturers quote a replacement recommendation of 3mm for wet-weather performance. By 1.6mm, wet stopping distances are substantially longer than when the tread was new.
The RFR codes a tester applies are 31194 (tread depth below the legal minimum) and 31189 (tyre condition — sidewall damage, ply separation, cords visible). Both are Major defects. Either one means the car fails.
The 20p coin check
No specialist tools needed. Take a 20p piece and insert it edgeways into the main tread groove with the outer rim of the coin going in first. The raised rim of a 20p is approximately 2mm from the edge of the coin to where the design begins.
If the outer rim of the coin is visible above the groove — if you can see any of that raised border — the tread is at or below 2mm in that spot. That doesn’t automatically mean it’s below 1.6mm, but it means you’re in the window where a calibrated probe at a test centre will give a reading you might not like.
Do this check at several points around the circumference, and at several positions across the width of each tyre. The inside edge of the tyre — reachable by putting your hand behind the wheel arch — is where wear develops first on most cars with even mild camber or tracking issues, and where most drivers never look.
The 20p test will catch tyres that are obviously illegal. It won’t catch a tyre sitting at 1.7mm on a reading that a probe will measure at 1.5mm. It is a pre-screen, not a pass/fail verdict.
What testers actually measure — and why they fail tyres above 1.6mm
A tester with a digital depth gauge is measuring to the nearest 0.1mm. There is no rounding. A reading of 1.5mm is a failure. A reading of 1.6mm passes — technically. But there are conditions where a tester will fail a tyre with a measured depth above 1.6mm.
Uneven wear across the width. A tester is required to check the tread depth across the central three-quarters, not just in the deepest groove. If the tread at the centre of the tyre is 3mm but the tread at a point two-thirds of the way across is 1.4mm, the tyre fails at that point regardless of what the centre reads. Uneven wear — from tracking, camber issues, or suspension problems — can create localised worn patches that a quick glance won’t reveal.
Cord visibility. If reinforcing cord is visible through the rubber at any point on the tyre — tread or sidewall — that’s an automatic Dangerous rating. Cords don’t need to be through the full tyre; any visible cord is a fail. This can happen on tyres with plenty of average tread but a single physical impact or scrub damage.
Professional judgement on borderline cases. A tester can note an advisory — a non-failure observation — on tyres between 1.6mm and 3mm. That advisory will appear on the test certificate. It doesn’t affect the pass/fail result, but it creates a paper trail. If a tyre reads 1.7mm and the tester issues a Major, they need a reason beyond “borderline” — but uneven wear across the width, a worn patch not visible on first look, or an unusual wear pattern all give them grounds.
Sidewall damage
Sidewall condition is a separate check from tread depth, covered under RFR 31189. A tyre can have 4mm of tread and still fail on sidewall damage.
Bulges are the most serious. A bulge in the sidewall means the internal structure — the cords and ply layers that give the tyre its shape under load — has partially failed. The outer rubber has separated or the cord has broken. The tyre is at risk of sudden, rapid deflation under load. There is no safe driving distance on a bulging tyre. A tester will rate this Dangerous.
Cuts and tears. A cut that exposes the cord structure underneath the outer rubber is a fail. A superficial scuff or scratch that hasn’t penetrated to the cord layer is not — but a tester will look carefully at anything that appears to go deep. If you can see white fibres through a cut, you’re looking at cord.
Cracking. Older tyres develop cracking in the sidewall rubber as the compounds dry out, particularly in the sidewall groove near the bead. Fine surface crazing is often an advisory. Deep cracking that reaches the cord layer is a fail. A tyre that’s more than seven or eight years old — regardless of tread depth — deserves a close look at the sidewalls and possibly a conversation with a tyre specialist about age-related deterioration.
Mismatched tyres on the same axle
A tyre that individually meets all the standards above can still fail if it’s paired with a different tyre on the same axle. The MOT Inspection Manual requires that tyres on the same axle are of the same structure — both crossply, or both radial. Mixing a radial with a crossply on the same axle is a fail.
Size mismatches on the same axle are also assessed. If one tyre is 205/55 R16 and the other is 195/60 R16, both may be serviceable individually, but the different rolling circumferences and handling characteristics make the pairing a problem. In practice, garages rarely mix sizes on an axle — but buying a single used tyre as a replacement for a puncture and fitting it opposite a different size is how this happens.
Asymmetric and directional fitment
Asymmetric tyres have an inward-facing side and an outward-facing side, marked on the sidewall. Fitting an asymmetric tyre the wrong way around — inward face outward — means the tread pattern is running in the wrong direction relative to the road. This can cause uneven wear and affect wet performance, but more relevantly for the MOT, a tester who notices incorrect fitment on an asymmetric tyre can fail it.
Directional tyres — which have a V-shaped tread pattern designed to rotate in a specific direction — are similar. The rotation direction is marked on the sidewall with an arrow. A directional tyre fitted on the wrong side of the car rotates backward and won’t drain water as designed. Again, a tester who identifies this has grounds to fail the tyre.
Both of these are unlikely to be spotted by a quick pre-test look unless you know what to check. If tyres have been swapped recently — corner to corner or side to side — confirm the sidewall markings before the car goes in.
Before the test
Check all four tyres with a £5 digital depth gauge (available from any motor factor or online retailer). A gauge gives you a reading to 0.1mm — there’s no reason to guess when the cost of guessing wrong is a failed test and a tyre replacement that could have happened at your own convenience.
Anything above 3mm: fine. Between 2mm and 3mm: budget for tyres in the next few months. Below 2mm: replace before the test. Below 1.6mm at any measuring point: you’re already driving on an illegal tyre.
For tyre replacement costs in your area before you hand the car over, the MOT cost estimator gives a current price range by region and centre type. Compare that against direct tyre supplier pricing — the margin on tyres at a test centre is real, and knowing what you should be paying before you walk in changes the conversation.
The 20p coin takes thirty seconds. The depth gauge takes two minutes per tyre. Both are worth doing before a tester does the same job and logs the result on the official record.