Car tyre sizes – they’re important, but why?


As a motorist in Ireland, are you aware that there were 159 fatalities on our roads in 2017, with many more seriously injured, causing untold misery for far too many families? While we all know about airbags, understand the importance of wearing seatbelts (in the front and back), and appreciate and respect the need for speed limits, the opposite is true when it comes to the most important car safety technology of all – tyres.

Alas, when it comes to driver safety, many Irish motorists tend to take their tyres for granted, with little or no regards to how important they are.

But think about it. No matter what vehicle you drive, and any number of gismos and safety features you have, at the end of the day it’s your tyres that actually make contact with the road. And if your tyres fail – because they’re worn, underinflated, overinflated, damaged, or the wrong size for your car – none of the other technology is going to completely keep you safe. Tyre safety is really important, and when it comes to avoiding an accident it can make a difference – all the difference in fact.

Improving Ireland’s understanding of tyre safety is part of Continental’s ‘Vision Zero’ driver safety initiative. It’s ultimate aim is to completely eradicate the tragedies that are caused by road traffic accidents. Their mantra speaks for itself – “zero fatalities, zero injuries and zero accidents”. In our latest tyre safety article, BestDrive by Continental look at one often over-looked topic: correct tyre sizes, and why it’s never a good idea to fit the wrong tyre size.


Mixing tyres is never a good idea

Here’s a scenario for you to think about. Your vehicle has recently had new tyres fitted, but – just your luck and thanks to some debris on the road – you sustain tyre damage and so it needs replacing. You don’t want to pay top prices for a replacement tyre, and you’re offered a cheaper one that looks “similar enough” to the other three, but in fact it’s not the same. It’s a slightly different size, but hey, all tyres look the same, right, and it’s going to save you a few euros, so what’s the problem..? Read on to find out…

Fitting mismatched tyres is a disaster waiting to happen

Continental is one of many of the world’s automotive and tyre experts that strongly recommend fitting four of the same tyres to your car – no matter which tyre brand you use. And by “same” Continental mean the same size – the right size for your car – the same tread pattern, and the same age. Why? Well it’s not because they want to sell you more tyres and make more money, no. It’s way more important than money.

The reason is because it’s been catagorically proven to be much safer fordrivers to do this. In fact it really matters for everyone’s road safety – drivers, passengers and fellow motorists alike.

So, what can happen if you don’t follow the tyre and road safety experts advice?

For starters, mismatched tyres can interfere with the accuracy of your speedometer, resulting in you seeing false readings, and leaving you unaware of how fast you’re actually driving. This will increase not only your chances of being stopped for speeding, but also the possibility of underestimating the time and length needed to brake and stop safely.

Unequal tyres also adversely affects your car’s steering, accelerating and general handling characteristics, leading to instability. You’ll notice there’s less grip – and as a consequence less control – because steering feels sloppy and loose. Mismatched tyres can mean it takes noticeably longer for your vehicle to get up to speed. This is particularly dangerous if you require an urgent, sudden burst of speed to evade any danger – such as a collision – and, arguably more importantly, cars fitted with mismatched tyres also take significantly longer to come to a safe stop – and stopping distances matter, particularly in cold, wet or icy conditions.

Another consequence of uneven, mismatched tyres is that long-term your vehicle’s wheel bearings are at risk of damage, as well as the possibility of destroying your car’s clutch.

Aren’t there some cars with different front and rear tyres?

Of course, such as rear-wheel-drive BMWs and Mercedes-Benz, which roll off the production line fitted with wider tyres on the rear axis, but it’s important to note that this isn’t a mismatch. Why? Because the same tyres are fitted on each axel and the car manufacturers – in partnership with tyre manufacturers – like Continental – have made an intentional decision to boost the vehicle’s traction on the rear driving wheels, so as to enhance driving dynamics. The cars in question – as well as the ‘OE’ (Original Equipment) tyres selected – have been designed specifically for this purpose.

So, what can you change with tyres – if anything?

As long as you correctly match identical tyres across each axle you can choose to fit tyres with a higher speed rating when, for example, you’re using your vehicle on a racetrack. The same is true if you have need to transport heavier loads. You can fit tyres with a higher load profile. This is particularly pertinent if you have need to pull a trailer of some sort, or caravan.

If you need to make a change like this, the best thing to do is to either check with your vehicle manufacturer, or with a tyre expert at BestDrive by Continental. Don’t forget that in most cases you’ll also need to update your insurer. Better to do so than risk invalidating your insurance, right?

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Get to understand your tyre sizes

It’s always worth checking your own car to familiarise yourself with the tyre size numbers on the sidewall. This way you’ll always know that you have the correct tyres fitted. In fact you may even find there’s a mismatch, and we’ve covered the consequences of that. Here’s a quick guide on how to understand the numbers and letters on your tyre sidewalls.

The first number (A) is the tyre width (in millimetres). Next up is the tyre profile (B), which is calculated on the basis of the tyre’s width. If you also see the letter “R” it simply denotes that your tyres have “radial” construction, which is good.

The third number (C) represents the wheel rim diameter. This figure is shown in inches. The final numeric character (D) is the load index figure. If you see, for example, “79” it means the tyre can safely bear a total weight of 437 kg. This one’s important to get right because if the load index is too low for your vehicle and what you’re using it for your insurance could in all likelyhood be invalidated in the event of an accident.

Finally, at the end of the sequence, there’s a letter (1) which shows the tyre’s speed rating (on a scale from A to Y). If, for example, you see the letter V, it means the tyre can handle up to 149 mph (not that you would ever drive your car that fast, right?).

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If you’re in any doubt about mismatched tyres, speak with the experts at BestDrive by Continental.

If you ask yourself “how do I check my car tyre size“ and are unsure which tyres are right for your vehicle, get in touch with your local BestDrive. Our experienced, professional team will provide you with impartial, expert advice about the right tyres for you, tyre safety, expert fitting services and much, much more. You’ll find your nearest BestDrive branch at the top of this page.

BestDrive by Continental – You drive, we care.