The tyre manufacturer says there is “no link” between tread depths at 1.6mm and increasing accident rates and highlighted that changing tyres at 3mm would cost fleets money and increase carbon emissions – especially as a tyre became more fuel-efficient as it wears.
Instead, Michelin is calling for a change to the tyre testing regime to reflect wet braking performance at 1.6mm.
However, RAC has highlighted that many tyre and safety experts believe the 1.6mm legal minimum is insufficient to guarantee safety with a minimum 3mm tread depth for tyre replacement recommended
Meanwhile, data obtained from the Driver and Vehicle Standards Agency by campaign group TyreSafe has revealed that dangerous or illegal tyres are the second most common reason for a vehicle MoT failure after lighting.
Tyres do not perform the same when new – and as a tyre wears, and the tread depth reduces, the difference in performance will change, and differences may be accentuated, according to Michelin.
That’s because tyre performance was affected by many individual characteristics; casing design, materials used, rubber compounds, tread design, shape of grooves and sipes etc.
Modern tyre technology makes it possible to provide high levels of performance and grip from new, and through all of the tyre’s life down to the legal tread wear limit, said the manufacturer.
With that in mind, changing tyres early – before they were fully worn – did not guarantee greater safety, and no current studies had established a direct link between accident levels and tyre tread depth, said the company.
Suggesting that tyres needed to be changed early – before the legal limit/tread wear indicator was reached – was akin to enforcing a form of planned obsolescence, said Michelin.
At present, tyre tests are carried out on new products, but there is no consideration given to how their levels of performance will change over time. Michelin has now raised that issue – the fact that the only factor for safety is tyre performance – not tread depth. It wants industry test bodies and consumer organisations to start comparing and testing tyres when they are worn to the legal limit.
Michelin said that as long as tyres were not damaged, the safety on dry roads actually improved as tyres got worn. The company argues that a worn tyre stops a vehicle more quickly in the dry than the same tyre when new.
Another surprising improvement in performance of a worn tyre over a new one is fuel consumption. As tyre tread depth reduces, the fuel economy of a vehicle will improve with one tank of fuel in five being used to simply overcome the rolling resistance of a vehicle’s tyres.
The rolling resistance of a tyre at the point of removal at the legal tread limit is 80% of that tyre in a new state. Therefore, keeping a tyre on the vehicle until the legal tread wear limit increases the time when it is in its most fuel efficient state, and reduces a driver’s fuel bill.
Michelin concludes: “[Fleets] should think carefully before changing tyres earlier than the legal tread limit as they will be removing the tyre when the dry braking performance and fuel efficiency will be at their peak.”
Michelin tests have shown that on wet roads, some worn tyres can perform as well as some new tyres, and that although the remaining tread depth is a factor in wet braking, the performance of the tyre, at all stages of its life, was more important.
RAC highlighted tests by UK technical organisation MIRA which found that, once tyres were below 3mm, stopping distances increase dramatically. The difference in wet braking distance between a tyre worn to 3mm and one worn to 1.6mm could be as much as 44%.
Whilst all tyres legally sold in Europe meet the minimum tyre labelling standard when new, Michelin says its tests have shown that the wet braking capabilities of some tyres reduced quickly when worn, and might fall below the ‘minimum standard’ requirement. However, it pointed out that some premium products not only met the criteria when new, they did so when worn to the legal tread wear limit.
Worn tyres can be dangerous in the wet because a tyre’s tread helps disperse water away from the contact patch between tyre and road. If there’s less tread depth, less water can be shifted, increasing the risk of aquaplaning and losing grip. In heavy rain, each tyre can shift one gallon of water every second, illustrating how hard tyres work. Therefore, the deeper the tread means tyres can work better, improving grip.
TyreSafe chairman Stuart Jackson said: “Regardless of legislation, drivers need to take their responsibilities to road safety seriously and carry out routine checks to stay tyre safe out on the roads.”