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Can I change my tyre size?

Kwik Fit | Wednesday 28th February 2018 1:26pm

tyre markings on a tyre sidewall

When you buy a car from new, that car will have a recommended tyre size based on the rims it is fitted with. If you were to replace those rims you could technically alter your tyre size, although bear in mind that if you do, your speedo will need to be recalibrated to compensate for this change. But what if you weren’t swapping your rims? Could you get away with putting a different size tyre on your car?

In this blog, we take a look at when you can and cannot change your tyre size.

Mismatched tyres are a no-no

We all suffer from bad luck occasionally, so consider this scenario. Your car has recently had brand new premium tyres fitted, but – because of some debris left on the road – one of these new tyres sustains damage and needs immediate replacement. 

Rather than opt for a straight like-for-like replacement, you’re offered a cheaper tyre that’s “similar enough” to the other three, though not actually the same. It’s a slightly different size, but hey, it’s going to save you a few quid. No problem right? Wrong.

Pitfalls to consider when fitting mismatched tyres

Many tyre experts including Continental strongly recommend fitting four of the same tyres to your car – and by “same” they’re referring to the same size (the correct size for your vehicle), the exact same tread pattern, and, ideally, the same age. They don’t suggest this because they want to sell more tyres. They do so because it’s proven to be much safer for drivers, their passengers, and other road users. 

If you ignore this advice, you’re exposing yourself to various problems:

  1. Firstly, mismatched tyres have a habit of throwing off your speedometer. That means inaccurate, false readings resulting in you not knowing how fast you’re actually driving. In turn, if you underestimate your speed and have mismatched tyres, your stopping distances may be compromised.
  2. Secondly, fitting mismatched tyres can adversely affect your steering, acceleration and the all-round general handling characteristics of your car. Uneven tyres can lead to less grip, and that means less control. Your vehicle will feel sloppy, and you may struggle to achieve shorter stopping distances on wet, cold or icy roads.
  3. Thirdly, mismatched tyres can mean it takes your car noticeably longer to get up to speed. This is especially dangerous if you find yourself suddenly needing a burst of speed to evade danger, such as a head-on collision. Perhaps more importantly, it also takes noticeably longer to come to a safe stop.
  4. Next, if one of your tyres is in fact wider than all the others, mismatched tyres will ultimately cause serious instability, especially at high speeds, and particularly in wet or icy driving conditions. This is of serious concern for drivers with uneven tyres driving on a motorway, where a sudden loss of control at high speed could prove catastrophic.
  5. Finally, long-term, if you fit mismatched tyres you are putting yourself at risk of damaging your car’s wheel bearings, and even destroying the clutch. Not only does this have implications for your driver safety, there’s financial consequences too.

Aren’t some vehicles fitted with different front and rear tyres?

While it’s true that some cars do have different tyres fitted on their axles, these are typically done deliberately at the vehicle manufacturer’s factory, in partnership with tyre makers. The cars in question are often rear-wheel-drive BMWs and Mercedes-Benz which roll off the production line fitted with wider tyres on the rear wheels. It should be noted that this doesn’t equate to a mismatch, since the same tyres are fitted on each axel, and the vehicle manufacturers have made an intentional decision to increase the traction generated on the rear driving wheels, so as to enhance the car’s driving dynamics. These cars – as well as the ‘OE’ (Original Equipment) tyres selected – have been designed and built to accommodate different tyres.

Are there any circumstances when I can change with my tyre size?

There are. As long as you match the exact same tyre type across the axles you can employ those with a higher speed rating. You might consider doing this if for instance you intend driving your car on a racetrack.You can also change your tyre size if you ever need to transport heavier loads. If you need to transport something heavy, or if you’re pulling a trailer of some sort, you can fit tyres with a higher load profile or are marked XL or RF meaning ‘Extra Load’ or 'Reinforced'. A word of caution though. If you’re intending to make changes like these, it’s probably a good idea to check with your vehicle manufacturer, or with a tyre expert at Kwik Fit. You should probably let your vehicle’s insurer know too, just to be safe.

When it comes to tyre size, do you know how to read your tyre sidewall?

It’s worth checking your own car to familiarise yourself with the numbers on your tyre sidewalls. Remember, they should all read the same. If you do so you’ll always be sure to have the right tyres fitted, and if you’re checking for the first time you may even find there’s a mismatch. Here’s a quick guide on how to understand the numbers and letters on a tyre’s sidewall.

tyre sidewall with labels

‘A’ represents the tyre width in millimetres. Next up comes the tyre profile (B) which is calculated on the basis of the tyre width. If you also see the letter “R” it just means your tyres have “radial” construction which is the standard for moden tyres.Next comes ‘C’, which is the wheel rim diameter. Traditionally, despite the millimetres being used for the other values, this figure is shown in inches. ‘D’ represents what’s known as the load index. For example, if you see the number “88” this means the tyre can safely bear a total weight up to 560kg. It’s particularly important to get this one right since if a tyre’s load index is too low for your vehicle and what you’re using it for your insurer could invalidate your policy in the event of an accident.

load index table

And finally, 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 ‘H’ it denotes that the tyre can handle up to 130 mph.

speed rating table

If you’re unsure about tyre sizes, talk to Kwik Fit

If you’re unsure which tyres are right for your vehicle, get in touch with your local Kwik Fit centre. Our tyre technicians can provide you with expert advice, answer your questions about tyre safety and much, much more. 


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