New vs worn tyres: how they compare

Everyone knows tyres don't last forever. They’re classed as a ‘wear item’, which means that, at some point, they’ll need replacing. Just like a pair of shoes.

Legally, tyres need to have a minimum tread depth of 1.6mm across the central three-quarters of the tread around the entire circumference of the tyre. Any lumps might mean the tyres are structurally unsound, and any cut bigger than 25mm (or 10% of the tyre’s width) could also mean your tyres aren’t legal. The same applies if any part of the ply or cord is exposed.

But what if your tyres meet all the legal requirements – they’ve just done a lot of miles and perhaps had a hard life in the process? How would they perform against a brand new set of rubber straight from the factory? We decided to find out.

What we did

Our long-term Audi RS3 had done just over 11,000 miles on the same set of Bridgestone Potenza Sport tyres. Most of those miles were racked up during normal road driving, although the car was also performance tested at our track and was involved in a drag race against a Kia EV6 (check out the What Car? YouTube channel if you want to know the winner). It was even borrowed by Chris Harris from Top Gear for a few days to skid around on some mud and crash through some cardboard boxes.

Yet despite all of this, the tyres had no damage to speak of and between 2.5mm and 3mm of tread depth remaining. So, with the old set of tyres still on the car, we conducted some acceleration, braking and handling tests to get a benchmark to compare with some new tyres. Tyre pressures were checked and adjusted before we started, and each test was run three times to
get a reliable average. We then switched to a new set of identical-specification Bridgestone rubber, set the correct tyre pressures again and then repeated the tests.

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The results

In all of our tests, the new tyres outperformed the old set. But are we talking about big enough differences for it to be worth you considering changing your tyres while they’re still well within the legal limit, at a potential cost of several hundred pounds?

Well, they shaved an average of 0.14sec off the 0-60mph time and almost 0.5sec off the lap time around our short handling track. The biggest difference, though, was in braking performance. With the new tyres fitted, the RS3 needed 3.45 metres (7.3%) less road to stop from 70mph. That could easily be the difference between escaping with sweaty palms and being involved in a collision.

We asked Bridgestone’s technical manager, Gary Powell, for his thoughts on the results. He told us that performance alters slowly as the tyre tread wears. In some instances, dry-weather performance can actually improve, but with more extreme cornering, braking and acceleration, performance can also decay. This is because the tread block surface area reduces, becoming rounded at the edges. In simple terms, the smaller the surface area of the tread blocks, the less rubber there is in contact with the road.

What about in the rain?

Although all of our tests were conducted in dry conditions, even bigger differences would most likely have been observed had it been wet. In fact, the whole reason why tyres have tread patterns on them is to displace water.

A reduction in tread groove volume means less water can be processed, and this is amplified in heavy rain or when driving at high speeds.If the tyre’s tread can no longer clear the water on the road in front of it, aquaplaning can occur. This means the tyre rides up on a wedge of water, so it’s no longer in contact with the road, and the car can’t respond to steering inputs.

Powell told us that data from other independent tyre tests has shown that wet braking performance decays exponentially below 3mm. As such, Bridgestone’s general recommendation is to replace tyres when the tread depth hits 3mm. However, he pointed out that some state-of-the-art tyres can maintain good wet performance down to just 1.6mm.

Verdict

While our expert acknowledged that wet-weather performance decreases rapidly at low (though still legal) tread depths, our tests show there can also be a sizeable reduction in dry conditions – particularly if you’re driving a high-performance car and make full use of it.

No, we’re not advocating throwing away a perfectly good set of tyres just to shave a fraction off your 0-60mph runs. But given that your tyres are the only part of your car that’s actually in contact with the road, we would recommend changing them more often than is legally required to maximise braking performance.

How to keep your tyres in good condition

Look for damage

Give your tyres a visual check once a week to ensure that there are no cuts or bulges in the sidewalls and that they aren’t wearing out unevenly.

Check the pressures regularly

Use a properly calibrated tyre pressure gauge to ensure that your car’s tyres are at their recommended levels. It pays to do this once a month when the tyres are cold. If the tyres are underinflated, their rolling resistance will be increased, and that means more fuel will be burned to push your car along. Underinflated or overinflated tyres will affect your car’s handling,  braking and grip, plus they’re more prone to punctures and will wear out quicker.

Check the wheel alignment

Knocking a wheel on a kerb or worn components can cause a wheel to go out of alignment, and this can result in either the steering pulling to one side or juddering at around 40-50mph. Ignore this and your tyres will wear unevenly and might need replacing sooner than you’d normally expect. A garage will be able to check the alignment and put it right if needed.

Drive smoothly

Try to drive at a steady speed, because heavy acceleration or braking will wear down tyre rubber more rapidly. Slow down over speed humps; hitting them hard can affect wheel alignment. And avoid potholes as much as possible, because they’re the main cause of tyre damage.

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