The days when a fleet manager wandered round the yard measuring tyres might be in the past, but the debate about tread depth isn’t.
The question is whether the legal minimum tread depth of 1.6mm across three-quarters of the width of the tyre is satisfactory.
Road safety organisations like Brake and RoSPA don’t think so.
Brake is lobbying the Government to raise the minimum tread depth to 3mm.
It argues that tyres worn below 3mm can be dangerous in wet or icy weather, and tests carried out by the Motor Industry Research Association (MIRA) appear to support this – they show that stopping distances greatly increase in wet conditions when tyres are worn.
MIRA’s tests involved four different vehicles and they all travelled on wet roads at 50mph and tyres were tested at four different tread depths.
The results showed a clear difference between tyres worn to 3mm and those at 1.6mm.
On 3mm depth the stopping distance was 31.7 metres whereas on 1.6mm tread it was 39.5 metres – nearly a bus-length extra.
- Aquaplaning risks
Richard Adams at MIRA says: “The debate has been going on for years and every time we do research the results are the same.
"The message is that you should look at changing your tyres long before 1.6mm.”
Continental is one tyre manufacturer which recommends changing tyres at 3mm.
Tim Bailey, safety spokesperson at Continental, explains: “A tread depth of 1.6 mm is not sufficient to channel the water away from underneath the tyre, meaning the tyre loses contact with the road and the driver can lose control of the vehicle.
“Although many cars come fitted with ABS and ESC to help the driver keep control, this has little benefit if the tyres are not in contact with the road.
“The research shows that the risk of aquaplaning becomes more pronounced when the tread depth goes below 3mm which is why we recommend changing tyres at this level.”
Chris Wakely, a representative of TyreSafe (formerly the Tyre Industry Council), also says fleet managers should consider changing tyres at 3mm.
“With 1.6mm a vehicle’s ability to stop in the wet is seriously compromised,” he says.
“I’ve done wet weather braking on vehicles with tyres of 1.6mm and found the braking distance could be up to double.
"Considering our roads are wet about 40% of the year, are you prepared to suffer the consequences of driving in the wet with worn tyres?”
Others argue that even though there is evidence showing a relationship between stopping distances and tread depth, more data needs to be provided of the effect on accident rates.
Fleet consultant Colin Tourick says: “If anyone is suggesting fleet managers should replace tyres at 3mm it is up to them to come forward with hard evidence that this would reduce accidents.
"I have not seen any such evidence.”
Tyre manufacturer Michelin agrees.
“The available accident data does not demonstrate any increase in accident rates on wet roads with tyres worn close to the legal limit.”
Michelin also points out that other factors influence wet weather braking, such as tread pattern design and tread compound – a view echoed by Goodyear.
“A tyre’s design is just as important as the depth of tread,” James Bailey at Goodyear says.
“Different brands have different grooves and suggesting that all tyres become ineffective at 3mm is a confusing message.
"Our internal testing shows there’s a big difference between different brands of tyres when worn to 3mm and we’d welcome independent testing looking at this.”
The driver’s behaviour, the road surface and the vehicle itself play a role in stopping distance, according to Michelin.
“The increased distance due to tyres with lower tread depths is only one factor that helps determine overall stopping distance, and alone it cannot be taken as an indicator of accident frequency.”
- Environmental impact
And what about the environmental impact of changing tyres at 3mm?
New tyres have more rolling resistance and this results in slightly more fuel consumption.
Michelin suggests that changing tyres at 3mm will result in increased fuel consumption of about 0.55%, which means an extra one billion litres of fuel and 2.5 million tons of CO2 emissions a year across Europe.
In addition, 23% more tyres would need to be manufactured, putting pressure on natural resources and increasing the number of scrap tyres. In turn, this will push up the price of tyres.
In fact, cost is one thing deterring people from changing tyres at 3mm.
Nigel Fletcher, operations director at ALD Automotive, points out that tyres are a big part of maintenance and replacing them at 3mm could increase costs by 20 to 25%, depending on vehicle mileage.
So how does a fleet manager decide when to replace tyres?
How do they find the balance between safety, cost and the environment?
Many leasing companies have decided the balance lies in replacing tyres at 2mm.
Steve Phillips, head of supplier development at Lloyds TSB Autolease, says: “Our policy is to change tyres at 2mm because customer safety is crucial and we want to do as much as we can to ensure that customers do not drive below the legal limit.
"Once tyres get to around 2mm of tread remaining, it’s difficult to determine how long it would take to hit the legal limit because tyres wear at different rates and would also be influenced by individual driving conditions.”
Lex suggests that a large number of its customers tend not to check tyres as regularly as they should.
“We’ve had a 2mm policy for quite some time and if our policy was 1.6mm it may mean too many customers allow tyres to run right to the edge,” explains a spokesman.
Kwik-Fit is Lex’s major supplier and it too has a policy of replacing tyres when they reach 2mm.
They’re not the only ones.
The Metropolitan Police has a change policy of 2mm across the whole of the tread as “it provides a greater margin of safety”.
So, if many organisations replace tyres before they wear to 1.6mm for safety, why has it been set as the legal limit?
According to a Department for Transport spokesperson it was an EU directive, set in 1989.
The problem, which road safety campaigners point out, is that in the late 1980s tyres were thinner and vehicles had less horsepower.
As tyres evolve, are the rules still suitable?
It’s a decision a fleet manager will have to make on their own.
The Government has no plans to change the rule.
One thing is certain, with the recent corporate manslaughter legislation a fleet manager can’t ignore the issue of tyre safety.
Perhaps it’s time to get back to checking the car park, armed with a tread depth gauge.
- Measuring tread depth
Tyre tread depth gauges are a simple and inexpensive way of being able to easily check your tyre tread depths on a regular basis.
They can be purchased from an automotive accessory retailer or from most petrol and service stations.
If you don’t have a tread depth gauge then the border of a 20p piece can be inserted into the main grooves to see if you tyres are below or above 3mm.
- Tyre facts
Two-thirds of drivers (68%) don’t know that the minimum tyre tread depth is 1.6mm, according to a recent survey by Brake and Green Flag.
Research from TyreSafe suggests 12% of cars and vans on the roads in the UK have at least one tyre below 1.6mm.
Fines for driving on defective tyres are harsh – each illegal tyre is treated as a separate offence. Four illegal tyres could result in a fine of £10,000 (£2,500 per tyre) as well as three points per tyre on a driving licence.
Research by Kwik-Fit Fleet suggests that 25% of inspected vehicles require immediate attention, usually because tyre tread depths are illegal.
A third of all road crashes resulting in injury are caused by under-inflated or defective tyres, according to Department for Transport figures.