Tyre tread depth is the single most common reason a UK car fails its MOT. Not brake pads, not bulbs — tyres. The DVSA’s own data puts it near the top of the Major defect list year after year, across almost every class of vehicle. Most of those failures are entirely avoidable with a thirty-second check in the driveway the week before the test.
The legal standard — and what it actually means
UK law requires a minimum of 1.6mm of tread depth across the central three-quarters of the tyre’s width, around the full circumference of the tyre. That’s the Motor Vehicles (Construction and Use) Regulations 1986, and the MOT tester is applying exactly that rule when they press a probe into the groove.
Three things in that sentence matter:
- Central three-quarters. The outermost edges of the tread block don’t count. The rule applies to the middle 75% of the tyre’s running surface.
- Full circumference. The tester checks all the way round. A worn patch at the twelve o’clock position is not hidden by good tread at six o’clock.
- 1.6mm minimum. This is the floor, not a target. Tyres are the only thing between your car and the road at every braking event. Most tyre manufacturers recommend replacing at 3mm. You are not rewarded for running it down to the wire.
The Reason for Rejection logged on a failed test is “Tyre tread depth not in accordance with the requirements” — you’ll find every car’s historical failure rate for that defect at /failures/tyre-tread-depth-not-in-accordance-with-the-requirements-31194/.
The 20p coin method
You don’t need any tools for a basic check. Take a 20p coin and insert it into the main tread groove. The outer band of the coin — the raised rim that surrounds the number — is approximately 2mm wide. If that outer band is visible when you insert the coin, your tread is likely under 2mm. If it disappears into the groove, you’re probably fine.
Do this at multiple points across the tyre’s width, and in several places around the circumference. Pay attention to the inside edge of the tyre — reaching in behind the wheel arch — because that is where wear tends to develop first, and where it’s most likely to be missed by a cursory check.
The 20p check is a rough indicator. It will catch tyres that are obviously illegal. It will not catch a tyre sitting at 1.7mm on a borderline reading that a tester’s calibrated probe will clock at 1.5mm.
Why testers fail tyres well above 1.6mm
This surprises people. A tester can — and frequently does — fail a tyre reading 1.9mm or 2.0mm.
The legal minimum is 1.6mm, but the tester’s job is to assess the tyre’s condition at the time of test. If a tyre is at 2.0mm now, has visible uneven wear across the width, and was clearly heading south before it arrived at the centre, a tester recording a Major defect is making a professional judgement that the tyre doesn’t meet standard — not applying arbitrary caution.
There is also an element of self-preservation in the tester’s position. Signing off a tyre at 1.7mm on a car that returns a week later with a blowout puts the tester in an uncomfortable position. Most experienced testers have a working threshold closer to 2.0mm before they feel comfortable passing a tyre without an advisory note. An advisory at 2mm, a failure at 1.6mm — that’s broadly the pattern you’ll see in practice.
Reading the wear pattern
Where your tyre wears tells you something worth knowing before the car gets in front of a tester.
Worn down the centre only — over-inflation. The centre of the tread carries all the load while the edges lift slightly. Common on cars that are chronically over-inflated against manufacturer spec.
Worn on both outer edges, crown raised — under-inflation. The opposite: the centre bows upward slightly and the edges carry the weight. Under-inflation is also harder on fuel and accelerates carcass deterioration.
Worn on one shoulder only — tracking or alignment. The wheel is running at an angle to the direction of travel. The inner edge wearing faster than the outer is the typical presentation, and the typical fix is a four-wheel alignment check before the MOT. Tracking is not an expensive job; a new set of tyres because you ignored the tracking absolutely is.
Feathered or sawtooth tread blocks — worn steering or suspension components. The blocks are scrubbing rather than rolling cleanly. This can come from worn tie rod ends, ball joints, or worn suspension bushes. The tyre wear is the symptom. The underlying cause will also fail the MOT.
Load rating and speed rating mismatches
A tyre that physically fits the car and has 3mm of tread can still fail the MOT if the load index or speed rating is lower than the vehicle manufacturer’s minimum. This is an increasingly common issue on cars where previous owners have fitted budget tyres without checking the spec sheet.
The required ratings are in your owner’s manual and on the door placard (usually driver’s door jamb). A tyre sidewall reads something like 205/55 R16 91V — that last pair of numbers is the load index (91 = 615kg per tyre) and speed rating (V = up to 149mph). If the car requires a 94W and you’ve got a 91V, a tester can refuse the tyre as unsuitable for the vehicle.
This gets flagged less often than tread depth, but it’s worth verifying before you buy a part-worn or budget replacement in a hurry.
Getting a calibrated reading
A digital tread depth gauge costs about £5 on Amazon UK (affiliate link — tag=motcost-21) and gives you a reading to the nearest 0.1mm. There is genuinely no reason to guess. Press it into the main groove, perpendicular to the tread blocks, and read the number. Do it at three or four points across the width and at three or four points around the circumference for each tyre.
Anything over 3mm and you’re comfortable. Between 2mm and 3mm, budget for tyres in the next few months. Under 2mm, sort it before the MOT. Under 1.6mm at any measuring point, you’re failing — and technically driving on an illegal tyre.
Before you book the test
Run the MOT cost estimator to get a sense of what a tyre replacement costs in your area before you hand the car over to a centre. Garages are not obliged to use your preferred tyre supplier, and the margins on tyre fitting are meaningful. Knowing the ballpark before you walk in protects you from the £140 tyre quote for a tyre that retails at £65.
If you check your tyres a week out and they’re borderline, buy them before the test. Failing on tyres is a straight retest, which is free within ten working days — but only if the car is retested at the same centre. Buying tyres elsewhere and taking the car back in is straightforward. Showing up to the original retest with tyres from a different supplier is also fine. The paperwork is about the test, not about who sold the rubber.
The tread check takes two minutes. The MOT failure does not.