Tyres and Tyre Pressure

Why tyre safety is essential

A car’s tyres are important in several ways, affecting safety, acceleration, braking, handling and ride quality. They are the only point of contact between the car and the road and must do a lot of work with every input a driver makes. It’s therefore essential to ensure that tyres are in good condition, with a good depth of tread at the correct pressure, all ready to do their job, gripping the road for safe, smooth motoring.

Tyre safety and the law

This is not just advice from a mechanic’s perspective, but it’s also the law, with stiff penalties underlining the dangers of driving with worn tyres. A fine of £2500 and three penalty points is applied for each tyre found to be below the legal minimum of 1.6mm, with the ultimate sanction of a ban and £10000 fine for having all four tyres below the limit.

While this is the legal limit in the UK and Europe, many experts would say that 3mm is more like a safe limit after which a tyre should be replaced, due to tyres critical role in safety. Worn tyres are significantly reduced in their ability to displace water in wet weather, meaning stopping times are increased. They can fail under heavy braking or at high speeds, all of which greatly increases the risk of accidents.

Taking action on tyre safety

Yet many drivers will still be driving with tyres that are below this limit, perhaps either unaware or actively ignoring advisories. Survey data from Halfords suggests up to 25% of cars on the road in the UK could potentially have one or more faults, much higher than official statistics.

Safety charity TyreSafe reports that, going by DVSA figures, 2.15 million vehicles failed MOTs in the 2023-2024 period due to defective tyres. Again, indicating that motorists are continuing to drive, perhaps in hope of making it to the next MOT.

More importantly, defective tyres are a major cause of accidents with police and motoring organisations and charities advising motorists to act with urgency on advice about the condition of their vehicle’s tyres – some suggest there should even be mandatory reminders issued after a tyre advisory.

Checking tyre tread

The RAC describe the check as being that “the 1.6 minimum tyre tread depth must be the case for the middle three quarters of the tyre”. So to check a car’s tyres are above this 1.6mm legal limit, techniques range from the “20p test” (the outer band of the 20p coin should not be visible when inserted into the tread), the use of a tread depth gauge, or what we would recommend – visiting a reliable tyre specialist to have the tyres professionally checked, and to benefit from professional advice.

Why do tyre pressures matter?

Tyres should be inflated to the correct pressure recommended by the manufacturer to ensure safety, performance, economy and handling are all optimal. The correct pressure is indicated on a plate typically found inside the driver’s door with the correct pressure according to tyre type and vehicle load illustrated. Correctly inflated tyres ensure optimal contact with the road and will wear evenly and predictably over a longer period.

Wrongly inflated tyres will wear unevenly and wear out faster meaning they need to be replaced sooner.

Under-inflated Tyres

Under-inflated tyres are a danger: negatively affecting handling, increasing stopping time and making the tyres more prone to damage and punctures. They wear unevenly on the edge of the tyre with the centre, which should be in touch with the road, bulging inward. The increased “rolling resistance” also increases fuel consumption.

Over-inflated Tyres

On the other hand, over-inflated tyres affect handling and wear by reducing the tyre’s contact patch with the road, placing stress on the tyre wall causing it to bulge outward. Grip is reduced by the increased wear on the centre of the tyre, which should offer most grip – when correctly inflated.

What causes loss of tyre pressure?

A car tyre may appear to be solid, but air can gradually escape in minute quantities over time due to osmosis or permeation, as soon as the tyre has been inflated. Changes in temperature, damage to the rim and faulty valves can also cause loss of tyre pressure.

The steady loss of pressure after inflating a tyre correctly can be a sign of a slow puncture. This may not necessarily mean the tyre needs to be replaced – minor damage can be repaired. Staff at a reliable local tyre centre, such as the tyre specialists at Dowleys, can advise on the best course of action.

Checking tyre pressure

While all new vehicles sold in the UK and EU must have a Tyre Pressure Monitoring System (TPMS) installed, it is still good practice to regularly check tyre pressures yourself, particularly in very cold or hot weather, or before starting a long journey. It’s a good idea to familiarise yourself with how to check tyre pressure on the TPMS. For older vehicles, or if you want a manual check, either pick up a tyre pressure gauge (reliable models available under £10), or make it a regular check when filling up at the petrol station. A once-a-month check is advised by the Department for Transport and leading tyre brands.