One such voice is the 3mm Tread Campaign (www.3mmtyres.co.uk) lead by Roadsafe and the National Tyre Distributors Association.
That campaign specifically relates to car, not truck, tyres. In the UK, the legal minimum truck -- greater than 3.5t gvw -- tyre tread is 1mm "in a continuous band throughout the central three-quarters of the tread width and over the whole circumference of the tyre," according to TyreSafe; the same rules apply to regrooved tyres.
Michelin says there is no link between car tyre tread depths at 1.6mm and increasing accident rates. In addition, changing tyres at 3mm would cost the motorist money and increase carbon emissions - especially as a tyre becomes more fuel-efficient as it wears.
An Ernst & Young report commissioned by Michelin found that changing tyres at 3mm instead of 1.6mm would cost European Union drivers an extra €6.9 billion a year in unnecessary tyre purchases and additional fuel consumption.
Instead, Michelin is calling for a change to the tyre testing regime to reflect wet braking performance at 1.6mm.
Michelin says: “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. This is because tyre performance is 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.
“With this in mind, changing tyres early (i.e. before they are fully worn) does not guarantee greater safety, and no current studies have established a direct link between accident levels and tyre tread depth. Suggesting that tyres need to be changed early (before the legal limit / tread wear indicator is reached) is akin to enforcing a form of planned obsolescence.
“Changing tyres too early would result in 128 million additional tyres being used a year in Europe - which would cause nine million tons of additional CO2 emissions every year. In addition to the environmental impact, replacing tyres before they are fully worn also represents a significant and unjustified increase in costs for consumers; Ernst and Young estimates an extra 6 billion euros in Europe alone.
Michelin argues that the issue is not tread depth but tyre performance. It says: “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 is now raising this issue – the fact that the only factor for safety is tyre performance - not tread depth. ‘The truth about worn tyres’ is calling on industry test bodies and consumer organisations to start comparing and testing tyres when they are worn to the legal limit.”
It also argues that in many places emergency braking is carried out in dry conditions. For example, citing Met office data, it points out that in London, roads are dry for 71% of days per year (106.5 days).
It continues: “The good news for motorists is that as long as tyres are not damaged in any way, the safety on dry roads actually improves as their tyres get worn. As seen on race circuits around the world, in dry conditions the ‘slick’ is the tyre of choice; and similarly for the ordinary motorist, levels of grip in dry conditions increase as the tyre tread depth reduces. A worn tyre will stop a vehicle more quickly in the dry than the same tyre when new. Although the differences in stopping distance are not huge, demonstrations on the test track at Ladoux show a definite improvement, a shorter stopping distance on worn tyres in the dry.
“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 the vehicle will improve, and with one tank of fuel in five being used to simply overcome the rolling resistance of the vehicles tyres, this is a welcome benefit. The rolling resistance of a tyre at the point of removal at the legal tread limit is 80 per cent 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 the motorist’s fuel bill.
Michelin moves on to consider braking in the wet. First, it argues that tests should consider lateral grip as well as braking. It says: “When demonstrating tyre performance, comparing different tyre brands and different stages of a tyre’s life, it appears that the majority of testing is basic straight-line braking. Why do we not see more lateral grip demonstrations?
“The simple reason for this is that it is relatively easy to measure, replicate and to quantify performance from wet braking tests, whereas the measuring of lateral grip and stability is very subjective and difficult to quantify. The good news is that wet lateral stability and wet braking are correlated. It is the same quality that is being tested, only the direction of the tyre travel that changes – one sideways/ laterally, the other in the direction of travel/ longitudinally. Demonstrations at Ladoux confirm that a better tyre in wet braking, is also a better tyre in wet cornering.”
Turning now to wet braking, it goes on to say that its own testing at Ladoux has found: “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, is more important.
It adds: “Whilst all tyres legally sold in Europe meet this minimum standard when new, Michelin tests have shown that the wet braking capabilities of some tyres reduce quickly when worn, and may fall below this ‘minimum standard’ requirement. However, some premium products not only meet the criteria when new, they do so when worn to the legal tread wear limit.
The tyremaker concludes: “With these findings that wet braking distances and lateral wet grip depend on the performance of a tyre and not solely the tread depth, Michelin is calling on industry test bodies, and consumer organisations to start comparing and testing tyres when they are worn to the legal limit; then consumers will start to discover the truth about worn tyres.”
UPDATED 4pm 22 May:
Added clarification that the 3mm campaign refers to automotive tyres and details of truck tyre tread limits are 1mm - information sourced from TyreSafe (see link below).