Incorrect Tyre Size: How It Affects Your Car’s Safety & Performance

Choosing the right tyres isn't just about tread pattern or brand preference — it starts with something much more fundamental: the correct tyre size. Every vehicle is engineered with precise dimensions in mind, and deviating from the recommended size can impact everything from braking distance to fuel efficiency. For drivers looking to stay safe and maintain performance, understanding tyre size isn’t optional — it’s essential.

If you're shopping for tyres in Coffs Harbour, it's worth knowing how the wrong fit could cost you more than just comfort. Here's why matching the correct tyre size to your vehicle matters — and how to avoid common mistakes.

Every vehicle comes with a manufacturer-recommended tyre size, carefully calculated to work with the suspension, handling system and safety features. Fitting tyres that are too wide, too narrow, too tall or too short can alter how your car drives — often in unpredictable or unsafe ways.

Tyre size affects performance in a number of areas:

• It determines how much of the tyre surface contacts the road, which impacts grip and stability.
• It influences steering response and the car’s ability to maintain traction in different driving conditions.
• It affects the alignment and balance of suspension components, which in turn affects ride quality.
• It changes the rolling diameter, which can throw off speedometer accuracy and gear ratios.

While it may be tempting to try a different size for aesthetics or perceived performance gains, it’s not always a safe or legal move without professional advice.

One of the first things you'll notice with incorrect tyre sizing is a change in how your car handles — and not for the better. Oversized or undersized tyres can interfere with your vehicle’s ability to steer and stop safely, especially during emergency manoeuvres.

Here’s how handling and braking can be compromised:

• Tyres that are too wide may rub against the wheel arch or suspension, causing vibrations or noise during turns.
• Narrow tyres can reduce your contact with the road, decreasing grip — particularly in wet or uneven conditions.
• Larger-than-recommended tyres may raise the vehicle's centre of gravity, making it less stable when cornering.
• Braking distances may increase if tyres can't maintain the necessary friction with the road.

These changes aren’t just inconvenient — they can pose real risks during high-speed driving or in sudden stop situations. If you’ve recently changed your tyres and noticed your car doesn’t “feel right,” it may be due to incorrect sizing.

Most drivers don’t realise that their tyre size also plays a role in how much fuel they use — and how accurately their car reports speed and distance. When you change the rolling diameter of your tyres (the total height of the tyre and wheel), it alters how the car’s systems interpret motion.

This can lead to several issues:

• A larger rolling diameter results in fewer revolutions per kilometre, which can make your speedometer read lower than your actual speed.
• Smaller tyres may cause the speedometer to overestimate speed, potentially leading to unintentional speeding.
• The engine may work harder with incorrectly sized tyres, especially if they affect transmission timing, leading to increased fuel consumption.
• Fuel efficiency can drop if tyre resistance doesn’t match what the car was designed for, particularly in stop-start traffic.

So, while bigger tyres might look good or feel smoother on highways, they could be quietly eating into your fuel budget — or causing legal trouble due to speedometer inaccuracy.

A poor fit doesn’t just affect the tyres — it impacts everything around them. Incorrect sizing creates uneven load distribution and can put unnecessary pressure on key components like suspension, wheel bearings and shocks. Over time, this stress can lead to expensive damage.

Signs that your tyres may be causing excess wear include:

• Uneven tread wear, especially on one edge of the tyre.
• Tyres that wear out faster than expected, despite regular rotation.
• A noticeable pull to one side while driving.
• Clunking or creaking sounds when going over bumps or during tight turns.

Not only does this mean replacing tyres sooner, but it may also shorten the lifespan of costly suspension parts. If you’ve recently changed tyres and are now noticing unusual wear, checking the size is a good place to start.

Legal & Insurance Implications in Australia

In Australia, vehicle modifications — including tyre changes — are subject to state and national regulations. Tyre size can’t be altered beyond certain limits without a formal engineering assessment or modification certificate. Failing to comply can not only result in fines but may also void your insurance if an accident occurs.

Here’s what you should be aware of:

• Tyres must stay within the overall rolling diameter variance allowed by law (generally no more than 15 mm increase or 26 mm decrease).
• Load and speed ratings must meet or exceed the manufacturer’s recommendations.
• Tyre changes that impact braking performance or ground clearance may require certification.
• Insurance claims can be denied if it’s found that incorrect tyres contributed to a crash.

Before making any changes, it’s best to consult a professional to ensure your new tyres are both safe and road-legal.

Choose the Right Fit With Expert Help in Coffs Harbour

With so much riding on your tyres — quite literally — it makes sense to get expert help when choosing replacements. Whether you're upgrading, replacing worn-out tyres or simply unsure if your current ones are right, getting professional advice ensures safety, compliance and performance.

By working with a trusted provider of new tyres in Coffs Harbour, you benefit from:

• Access to your vehicle's manufacturer specifications and size recommendations.
• A wide range of brands and models, tailored to your car type and driving habits.
• Proper fitting and balancing to prevent vibrations and premature wear.
• Advice on tread type, load rating and tyre lifespan.
  

Get Tyres That Fit Right & Perform Better

When it comes to safety, performance and long-term cost, tyre size is one detail you can’t afford to overlook. Fitting the wrong size might seem minor, but it can affect your handling, braking, suspension, fuel economy and even your legal compliance.

At GK Denney Tyres, we specialise in supplying and fitting tyres in Coffs Harbour that meet both your needs and your vehicle’s design. Whether you're after standard replacements or need guidance for a custom fit, our team is here to help.

Contact us to book a fitting or speak to our team for tailored advice. Get back on the road with the right tyres — fitted for safety, comfort and performance.