Common questions about tyres answered

What’s my tyre size and how do I read it?

Your tyre size is a full code on your tyre's sidewall. It's a sequence of numbers and letters which provides its dimensions and performance specifications.

You should find the width (mm), profile (percentage), and rim diameter (inches).

Each section of the code refers to a different part of the tyre.

Let's take a look at how you'd read each section if your tyre code read 205/55R16 91V.

• 205 (Width) is the width of the tyre in millimetres
• 55 (Aspect Ratio/Profile) is the height of the sidewall is 55% of the width
• R (Construction) stands for Radial construction (standard for most cars)
• 16 (Rim Diameter) is the diameter of the wheel rim in inches
• 91 (Load Index) is the numerical code for the maximum load the tyre can carry
• V (Speed Rating) indicates the maximum speed, with V corresponding to 149 mph (240 km/h)

How to find your correct tyre size using a registration check or calculator

There are a number of convenient online tools that can find your correct tyre size.

You can enter your vehicle registration on sites like Kwik Fit, Blackcircles, or ATS Euromaster. These sites can quickly identify your car manufacturer's specified size.

You should still check this against the numbers on your tyre sidewall, or wherever your tyre's dimensions can be found.

Where else can I find my car's tyre size?

Other than the tyre's sidewall, you can find the dimensions on a sticker inside the driver’s door jamb, in the fuel cap, or in your owner's manual. You may even be able to find this information in the glove compartment.

It may also be listed in your vehicle’s documentation, such as the certificate of conformity (CoC).

What are the load index and speed rating markings?

These essential codes indicate the maximum weight and speed your tyre can safely handle, and must be matched to your vehicle's requirements.

1. Load index: This code shows the maximum load (weight) a single tyre can support when inflated. For example, a load index of 91 means the tyre can carry a maximum of 615 kg. Passenger vehicle load indices typically range from 62 to 126.
2. Speed rating: This letter, located immediately after the load index, shows the maximum speed at which a tyre can safely carry its rated load.

Common speed ratings include:

• H: 130 mph / 210 km/h
• V: 149 mph / 240 km/h
• W: 168 mph / 270 km/h
• Y: 186 mph / 300 km/h

Higher ratings generally indicate better high-speed performance and handling.

What happens if you change your car's specification?

Changing your car's specification affects many aspects of its performance.

Minor variations within manufacturer-approved limits (often within +1.5% to -2% of the original diameter) are generally accepted, but larger deviations can affect safety.

Switching from the manufacturer's recommended tyre size can lead to:

• Speedometer errors: A larger tyre travels further per revolution, meaning your speedometer will show a speed lower than your actual speed. If the new tyre is smaller, your speedometer will show a speed higher than your actual speed. Being unaware of your speed is dangerous and can lead to speeding fines or even legal action.
• Reduced handling: Wider tyres generally provide more dry-weather grip and stability, particularly around corners, but they can increase the risk of 'tramlining', which is when the wheels follow ruts in the road. It can also mean reduced resistance to sliding on water, known as aquaplaning.
• Premature wear: If the tyre diameter or width is too large, it may rub against the wheel arches or suspension components, especially when turning or hitting bumps. Premature wear means you'll need to replace tyres more regularly, which can be costly.
• Damage to drivetrain components: Using different diameter tyres, especially on four-wheel-drive vehicles, can damage the central differential.

How often should you check your tyres?

You should check your tyres at least once a month to ensure the tread depth and pressure meet legal and safety requirements. It's also a good idea to check them before any long journeys or when your car is carrying heavy loads.

When checking your tyres, look at the pressure, tread depth, and general condition, making sure they're roadworthy. You should only drive if you're happy that your tyres meet the legal requirements.

When should I replace my tyres? What's the legal tread depth limit?

Tyres must always be replaced when the tread depth falls below the legal limit. This is 1.6mm across the central three-quarters of the tyre, around the entire circumference.

The 20p test is a quick and easy way to check your tyre's condition for safety and MOT compliance - simply insert a 20p coin into the main grooves of the tyre and check if you can see the outer band of the coin. If it's visible, the tread is likely below 1.6mm, making it unsafe and illegal.

Why is correct tyre pressure so important?

Correct tyre pressure is crucial for ensuring your car is roadworthy. This means your car is in a safe, legal, and operational condition to be driven on public roads.

Let's take a look at why the right tyre pressure is so important:

• Safety and handling: When your tyres are inflated correctly, they have better grip and stability on the road. Under-inflated tyres can make steering feel heavy, whilst over-inflated tyres can limit steering control. This can be particularly dangerous in wet or icy conditions.
• Improved fuel efficiency: Incorrectly inflated tyres can lead to increased rolling resistance, making the engine work harder and increasing fuel consumption.
• Shorter braking distances: The right pressure is needed for maximum traction, especially in wet or icy conditions, to keep braking distances short.
• Longer tyre life: When your tyres have the correct pressure, wear is spread evenly across the tread. Under-inflated tyres may wear out at the edges, whilst over-inflated tyres wear in the centre, leading to more frequent replacements.
• Reduced risk of blowouts: Excessive heat buildup can occur when tyres are under-inflated, increasing the risk of blowout.

Can having the wrong tyre size or illegal tread invalidate my car insurance?

Yes, having the wrong tyre size or illegal tread can invalidate your car insurance, leading to major financial consequences.

Unsuitable tyres make your car unsafe and illegal to drive. Your car insurance policy requires your car to be in a 'road-worthy' condition, so if tyre size and tread aren't correct, your policy could be void.

The car insurance provider may refuse to pay for any repairs on your vehicle, leaving you to front the costs in the event of a car accident.

Fitting tyres with different sizes or lower speed/load ratings than recommended is considered a car modification. If you don't inform your insurer about these modifications, or if the tyres make the car unsafe, your insurance could also be invalidated.

What are the fines and penalties for driving with illegal tyres?

Driving with illegal tyres can lead to serious legal consequences, including fines of up to £2,500.

You can also face penalty points for driving on unsafe or illegal tyres, and have your car insurance policy voided.

Check your tyres every month to ensure they're roadworthy, and to give yourself peace of mind before you drive.