How to keep your tyres safe

Thousands of accidents a year on Britain’s roads could be prevented if the minimum tyre tread depth is increased by 1.4mm, according to the shock findings of an Auto Express investigation. In the first study of its kind, we compared how much more quickly a car fitted with tyres that have 3mm of tread left on can halt from 70mph in the wet, as opposed to when the rubber is on the 1.6mm legal limit.

Our test involved performing an emergency stop while driving along a soaked tarmac test track. The results (see box) are horrifying, and show that the state of your car’s tyres can mean the difference between avoiding an obstacle or hitting it at 50mph.

It’s all down to the fact that braking performance in the wet drops off significantly once the tyre’s tread level falls below 3mm. We found it can take a staggering 44 metres - about 10 car lengths - more for a motor to stop at motorway speeds in the rain if its rubber is on the legal limit.

Worryingly, there are plenty of vehicles on our roads that have less than 3mm of tread. We randomly checked 100, and found 67 had at least one tyre that was below this safety threshold. It’s little wonder, then, that Government statistics show one-fifth of the 98,845 serious injuries and fatalities resulting from motoring accidents in wet conditions during 2004 involved cars skidding.

Obviously, if the legal limit was increased, we’d all have to replace our rubber more regularly. But according to one leading tyre maker, this would add only £20 a year to the cost of motoring for the average driver. That’s just 39p per week, and a small price to pay to improve road safety.

You could argue the minimum depth should be even higher, as the more tread the better. However, in 2003 an independent study by motoring research centre MIRA showed that around 3mm was the critical depth. Tests were carried out while driving at 50mph, but until now no research has been done at motorway speeds.

So, to corroborate this previous evidence, we also brake tested a Ford Focus on tyres with 5mm of tread. On average, it took 82 metres to come to a halt - nine metres sooner than when fitted with 3mm rubber. However, with the tyres on the legal limit, the stopping distance increased significantly to a scary 135m.

Chris Wakley, from the Tyre Industry Council, a safety organisation supported by manufacturers and retailers, said: “These new findings are shocking. They show for the first time how dramatic the reduction in braking performance can be between tyres with 3mm and 1.6mm of tread at motorway speeds in the wet. It also indicates that a change in the law is certainly worth considering.”

He continued: “Sadly, most drivers don’t realise how important the condition of their tyres is. They often only find out too late; when they are braking to avoid an obstacle and the car doesn’t stop.” Cynics might argue that it’s in tyre companies’ interest for drivers to change their rubber more regularly. But other groups back the move, too. The Royal Society for the Prevention of Accidents (RoSPA) is one of the biggest names in road safety, and its experts believe a change in the law would cut motoring deaths and injuries.

The same is true for Oxfordshire-based training firm Drive & Survive. A spokesman told us: “You can dramatically reduce the likelihood of aquaplaning by changing tyres when they reach the 3mm level. At the legal 1.6mm, you’ll have little or no wet-grip performance.”

This sinister warning is borne out by testing. Twelve months ago, the AA Motoring Trust produced a report entitled Get a Grip. One of the key conclusions of the research was: “Replace your tyres soon after the tread depth reaches 3mm. Always change them once the tread has worn down to 2mm or less at any point.”

Several vehicle manufacturers also recommend 3mm as the standard changeover point. German tyre firm Continental did some research in 2003 and 2004 to see which car handbooks made that suggestion as part of their advice to drivers. Ford, BMW and Mercedes did, and it’s also an aftermarket recommendation from VW and Porsche.

So if all the evidence points to the fact that 1.6mm isn’t a safe limit, why is this the law? The rules date back to 1992, when new cars had skinnier wheels that were smaller in diameter. Prior to this date, it was only 1mm. Continental spokes- man Tim Bailey explained: “Tyres have got fatter and bigger, which means the contact patch with the road is greater. In dry weather that’s fine, as it equals more grip, but in the wet it’s bad news. It means you need less water pressure over that bigger area to lift the car up and start aquaplaning.”

Think of it like this: water skis have a small surface area, and you need to move quickly to stay afloat. But surfboards are larger, and you don’t have to be going very fast to stay on the waves. Continental’s Bailey added: “That means on modern tyres, 1.6mm of tread often isn’t deep enough to channel sufficient water away from the road surface, whereas 3mm is.”

With so many motoring and safety groups believing a change in the law would save lives, why doesn’t the Government do something about it? Sadly, it’s not as simple as getting ministers to put legislation before Parliament. A Department for Transport spokesman told us: “The 1.6mm minimum tread depth for car tyres is set across the EU by a European directive. There are no plans to alter this, but it’s a policy we keep under review.”

So is it only the Brits who want the law to be updated? No. The TUV is the German equivalent of Britain’s Transport Research Laboratory (TRL), and recently published a report looking at the adequacy of existing rules. When talking about ways of reducing the number of accidents, it said: “A measure that appears useful is to raise the required remaining tread depth... especially with regard to wet-grip performance. An increase to 2.5mm for regular tyres is expected to result in a clear improvement in driving safety.”

Indeed, according to the UK Tyre Industry Council, German motorists tend to be more conscious of tyre safety than in Britain, and replace their rubber before it wears down to 1.6mm. But amazingly, some of the cast-offs are then shipped to the UK and sold here as part-worns.

Ultimately, updating the legislation depends on the Eurocrats in Brussels - but that’s no excuse. We believe the British Government should take a lead and call for a review - we’ve learned it will shortly be conducting its own research into the matter. In the meantime, you can be a responsible driver and change your tyres at 3mm anyway. After all, as our study shows, it could be the difference between life and death.

Stop watch: How we compared the performance of different tread depths

For our exclusive study, we visited Mira’s proving ground in Nuneaton, Warwickshire. The facility has a wet-weather test track which is covered with asphalt and offers similar grip and drainage properties to a motorway surface. On either side, sprinklers simulate the effect of rainfall.

Each of our cars was fitted with tyres ground down to exactly 3mm using a painstakingly accurate technique. We performed four brake tests for each model, using Racelogic VBox timing equip-ment to measure stopping distances from 70mph under full ABS braking to the nearest cm. The experiment was repeated with the vehicles swapping tyres for the same brand - this time ground down to 1.6mm of tread depth. A fifth test at 5mm was performed for the Ford Focus.

The Racelogic kit also allowed us to gauge how fast each car on rubber with 1.6mm of tread was travelling at the point when it would have come to a halt if fitted with 3mm tyres. The figures (below) are shocking, but don’t convey quite how alarming it is to perform an emergency stop from 70mph in the wet. Fortunately, we made this discovery at the test track, and not on a busy motorway.

Braking distance variations from 70mph in the wetTest Car     Stopping Distance 3.0mm tread     Stopping Distance 1.8mm tread     Variance     Speed of 1.8 car when 3.0 car stoppedRenault Clio 1.4     113m     151m     38m     50mphFord Focus 1.8     91m     135m     44m     50mphToyota RAV4 1.8     118m     155m     37m     50mphAudi A4 2.0     97m     127m     30m     40mph