If you are looking to buy new tyres, then knowing the required sizes and dimensions is essential.
Read our latest tyres guide to learn more about how the sizing works and the difference between your front and rear tyres.
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Tyre size explained
- Width (e.g. 225): The tyre’s width in millimetres
- Aspect ratio (e.g. 45): Sidewall height as a percentage of width
- Construction (R): Radial (standard for most cars)
- Wheel diameter (e.g. 17): Size of the wheel in inches
- Load index (e.g. 91): Maximum weight the tyre can carry
- Speed rating (e.g. W): Maximum safe speed for the tyre
How to check your tyre size
You can check your tyre size quickly by looking at the markings on the sidewall of your tyre. This is the most common and reliable method.
The size appears as a sequence of numbers and letters, for example: 225/45 R17 91W.
Each part has a meaning. The first number (225) shows the tyre width in millimetres, the second number (45) is the aspect ratio (the tyre sidewall’s height as a percentage of its width), ‘R’ indicates radial construction, and the number after it (17) is the wheel diameter in inches.
The final numbers and letter refer to the tyre’s load index and speed rating – how much weight it is designed to carry, and the maximum speed it can be driven to safely.
You can also find tyre size information inside your car. Check for a sticker located in the driver’s door frame, inside the fuel flap, or sometimes in the glovebox.
Your vehicle handbook will also list the correct tyre sizes and recommended specifications, including tyre pressures.
In the UK, another easy option is entering your vehicle registration on a tyre retailer’s website, which will display the correct tyre size for your car.
Always ensure replacement tyres match the correct size and ratings – and double-check markings to ensure correct fit and safe driving.
Aspect ratio
Aspect ratio is the height of a tyre’s sidewall and is displayed as a percentage of its width.
It appears as the second number on the tyre size sequence – XXX/XX.
When it comes to the number itself, 35-45 is typically a thinner sidewall, mostly associated with sportier models and tyre types.
Whereas numbers ranging up to 70 are for more comfort or off-road focused vehicles.
Tyre width
This is the first three-digit number that appears on the sidewall of the tyre.
The number shows the width from sidewall to sidewall in millimetres.
Wider tyres are for better grip and handling, but can reduce fuel efficiency and cost more to purchase.
Tyre diameter
After the letter ‘R’ (for rim), the following number is the diameter of the wheel in inches.
You must match the tyre diameter to the wheel size, otherwise it will not fit the vehicle correctly.
Larger diameters often improve handling, but smaller diameters can offer a more comfortable ride in combination with smaller diameter wheels.
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How to read your tyre size
Here is what to look out for when it comes to reading the wall of a tyre – make sure it matches what is required for your vehicle.
Can my front and rear tyres be a different size?
Yes, your front and rear tyres may be different sizes, but only in vehicles that are designed that way.
The best way to check, is to read your car owner’s manual or find your make and model on the OEM’s official website.
Many performance, sports, or rear-wheel drive cars will use this staggered tyre system, where the back two tyres are larger than the front ones.
However, fitting different tyre sizes to what is approved by the manufacturer will negatively impact the vehicle – including the steering, braking, and overall stability.
You can even cause electronic malfunctions or mechanical damage by fitting the incorrect size of wheel and tyre, as the wheel will be spinning faster or slower than the car’s systems expect it to be.
Make sure the same size tyres (with the same tread profile) are on the front two, and if they are different at the back, those rear tyres should match.
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