Car tyre sizes. Why are they so important?

Car tyre sizes. Why are they so important?

15 Feb

By Tom Boote

Did you know that approximtely 1,800 people are killed each year on Britain’s roads, with thousands more also seriously injured? That’s untold misery and person tragedy for far too many UK families.

While most of us are aware of vehicle safety features, like airbags, and understand why it’s vital to wear seatbelts – both in the front and back of the car – unfortunately most drivers still don’t fully appreciate the single most important driver safety technology of them all – tyres – and as a result take them for granted.

But if you take a moment to think about it – and you should – you soon realise that although you can have all the technological gismos and safety features you like come with your car, it’s actually only your tyres that make contact with the road.

And if your tyres fail – perhaps because they’re too worn, or underinflated, maybe damaged, or even the wrong size for your car – none of the other driver safety technology will help you. Tyre safety realy matters – more than anything else – and it can make a difference – in fact ALL the difference.

Just as at Protyre, improving driver understanding of tyre safety is part of premium tyre manufacturer Continental’s Vision Zero” initiative. Continental’s ambitious target – “zero fatalities, zero injuries and zero accidents” – is to eradicate the tragedies that are caused by road traffic accidents, not just in the UK but throughout the world.

In our latest tyre safety article, Protyre look at one area often over-looked – tyre sizes – and why it’s not a good idea to fit the wrong size.


Mixing tyres

So why would anyone choose to fit an incorrectly sized tyre? Consider this: your car has recently had a set of new tyres fitted, but – just your luck – one of these new tyres has a bad puncture and needs replacing. Someone offers you a cheaper tyre that’s “similar enough” to the others, although not the same. The tyre’s a slightly different size, but who cares, it’s saved you fifty quid. What’s the problem? Read on…


Pitfalls of fitting mismatched car tyres

Premium tyre manufacturer Continental is one of many automotive and tyre experts that recommend fitting four of the same tyres to your car. Period. By “same” they mean the same (right) size for your car, with the same tread pattern, and that are the same age.

They don’t make this recommendation because they want to sell more tyres, and make more money. No, it’s far more important than that. They do so because it’s been irrefutabley proven to be much safer for drivers to do this. In fact it really matters for everyone’s safety – passengers and fellow motorists alike.

So what is likely to happen if you don’t follow the tyre experts’ advise?
  1. Fitting mismatched tyres can often throw off your speedo, giving you an inaccurate reading leaving you unaware of how fast you’re driving. Risk of a speeding fine.
  2. Unequal tyres can adversely affect your steering, your acceleration, and general handling characteristics. You have less grip with the road and less control because steering feels noticeably sloppy. Risk of an accident.
  3. Having unequal tyres can result in it taking longer for your vehicle to get up to speed. This can be dangerous if you need to quickly accelerate to avoid danger, and  perhaps more importantly it can also take much longer for you to come to a safe stop. Risk of collision.
  4. If one of the tyres fitted on your vehicle is wider than the others it will typically lead to serious instability, particularly in wet or icy conditions. Risk of losing control.
  5. In the long-term, having mismatched tyres will increase the risk of damaging your vehicle’s wheel bearings, as well as possibly destroying the clutch. Risk of behicle damage, and the subsequent costs of replacement parts and repairs.


Don’t some cars have different front and rear tyres?

It’s true. There are some cars – usually rear-wheel-drive Mercedes-Benz and BMWs – that come from the factory specifically fitted with wider tyres on the rear wheels. It’s important to remember that this doesn’t equate to a mismatch, because the same tyres are fitted on each axel, and the manufacturers have made an intentional decision to boost the traction on the rear, driving wheels, so as to enhance driving dynamics.
If you’re car isn’t designed this way, don’t mix your tyre sizes. The cars in question – as well as the OE (Original Equipment) tyres selected – have been designed specifically for this.


So, what can you change?

So long as you match the same type of tyre across both the axles, you can fit tyres with a higher speed rating if you’re using your car for fun, such as at a licensed racetrack. The same is true if you need to transport heavier loads. In this instance you can fit tyres with a higher load profile, especially important if you’re planning on pulling a trailer or caravan.

If you need to make a change like this it’s important that you check with your vehicle manufacturer, or with a tyre expert at Protyre. In most cases, you will also need to notify your vehicle’s insurer. 


Get to know and understand tyre sizes

It’s worth checking your own car to familiarise yourself with these numbers. This way you’ll ensure you always have the right tyres fitted. You may even spot a mismatch.

Take a look at this simple guide to better understanding the numbers and letters.

The first number (A) is the tyre width in millimetres. The second number shows the tyre profile (B). This is calculated on the basis of the tyre width. If you see the letter “R” it simply means that your tyres have “radial” construction, which is good.

Next up comes the wheel rim diameter (C). This figure is shown in inches. The last number (D) is the load index. For example, if you see “79” it means the tyre can safely bear a total weight of 437 kg. This one’s particularly important to get right because if the load index is too low for your vehicle – and specifically what you’re using it for – your insurance could well be invalidated in the event of an accident.

Finally, there’s a letter character (1). This represents your tyre’s speed rating on a scale from A to Y. If you see V, that means the tyre can safely go up to 149 mph maximum, but of course you would never push it that far, right?




Unsure about mixing tyre sizes? Talk to Protyre

If you’re still unsure or confused about which tyres are right for your car, contact your local Protyre professionals. We‘ll provide impartial, expert advice about the right tyres for you, tyre safety, expert fitting services and more. Find your local Protyre by clicking the button below:

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