To stay safe on the road, using tyres that are the right size for your vehicle is essential.

Driving with mismatched tyres can have a negative effect on your steering, accelerating and handling of your vehicle - and can even invalidate your insurance.

Want to learn more about tyre sizes? You've come to the right place...

What Does Tyre Size Mean?

Your tyres have markings on the sidewall which can tell you size information about the tyre, in the form of letters and numbers.

If you want to find out the size of the tyres you’re currently using, you can learn how to inspect the tyre markings yourself.

The markings on the sidewall can tell you the following about your current tyres:

  • Tyre size

  • Width

  • Aspect ratio

  • Tyre construction

  • Wheel diameter

  • Tyre load capacity

  • Speed rating

When you inspect your tyres, you will also find other useful information, such as the brand name, manufacturing date code, and tread wear indicators in the grooves of the tyre.

You can refer to your vehicle’s handbook or the sticker on the driver’s side door to find out the correct size of tyres for your vehicle, too.

How to Read Tyre Size

Reading the markings on your car's tyres is actually quite a straightforward process.

Take the example we’ve chosen in the illustration here:  205/55R/16 91V.

The first three numbers refer to the width of the tyre, and the two numbers that follow the slash can tell you the aspect ratio. This is the sidewall height expressed as a percentage of the tyre's width. Both of these are in millimetres.

The letter that follows this refers to the type of tyre you own. Usually this letter will be ‘R’ for radial - this is the type of tyre that is most common. The letter could also be ‘B’ for bias belt, or ‘D’ for diagonal, if these tyres have been built differently.

The next two numbers show the diameter of the wheel rim, measured in inches.

The final two digits indicate the tyre's load index, which shows the maximum weight a single tyre can safely carry.

The concluding letter can tell you the speed rating for your vehicle. This is the top speed that you can drive at before your tyre becomes unsafe.

 

An infographic showing a tyre sidewall.

 

What Is the Right Tyre Size for My Car?

Some tyres have markings which indicate they are specifically designed to be used in a certain vehicle. 

An OE marking - which stands for original equipment - signifies that this particular tyre was designed by a tyre manufacturer to be fitted as standard, as supported by your vehicle manufacturer. 

If your vehicle was first fitted with OE marked tyres, then it is often recommended to replace them with OE tyres - but equivalent non-OE tyres that meet manufacturer specifications can also be suitable.

What Happens if My Tyres Are the Wrong Size?

Tyres that are too wide may come into contact with the strut and spring and damage your suspension.

Meanwhile, if you increase the tyre sidewall, you run the risk of your tyre coming into contact with the wheel arch liner when travelling at speed.

That’s why it is always best to speak to a professional mechanic if you have any plans to change the size of your tyres, especially if you were planning on going above or below the tyre size recommendation in your owner's manual.

Speak to a professional garage before you change your tyre size, so that they can advise you on what is best for your vehicle.

For your safety, it is highly advisable that you choose four matching tyres, which you should have fitted as pairs across each axle.

If you don’t match your tyres, you run the risk of having less control of your handling, steering, and you increase the chance of your vehicle becoming unstable to drive.

 

Can You Change Your Car Tyre's Size?

Generally speaking, it is not advisable to fit tyres that are different sizes to your vehicle.

If you fit your vehicle with mismatched tyres, this can lead to:

  • Inaccurate speed readings

  • Impacted steering

  • Sluggish acceleration

  • Compromised handling

  • Less grip on the road

  • Increased risk of accidents

  • Increased risk of damage to wheel bearings and clutch

There are some instances in which it might make sense to change your tyres.

For instance, you may wish to fit tyres with a higher speed rating if you're using your car at a licensed racetrack.

You must match the same type of tyre across both axles if you are planning on making a change like this.

Another example could be fitting tyres with a higher load profile if you know your vehicle will be used to pull a trailer or caravan.

If you are planning on making a change like this, you should check with your vehicle manufacturer or with a professional mechanic to ensure you're making the right decision for your vehicle.

Bear in mind that using the wrong tyre size, speed rating or load rating for your vehicle can affect your insurance - so you should check if you need to notify your insurance provider if you're planning on making any changes to your tyres.

Now that you know more about tyre sizes, you can make sure your vehicle is properly equipped.

We've made it easier than ever to compare local tyre providers, so you never have to worry about the condition of your car's tyres.

Choose the right tyres for your car so you can keep driving with confidence.

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