Tyre Size

Selecting the Right Tyre Size for Your Car

When your tyres become old, damaged, or worn out, it’s time to replace them. Choosing the right replacement tyres is vital for safety, performance, and fuel efficiency.

There are three key factors to understand when buying new tyres:

1. Tyre size

2. Load index

3. Speed rating

Making the correct selection ensures your vehicle handles properly and performs as intended by the manufacturer.

Where to Find Your Tyre Size

Luckily, finding your vehicle’s recommended tyre size is easy. You can locate it:

• On the inside of the driver’s door frame

• On the fuel flap

• Inside the glove compartment

• Or in your vehicle’s registration documents

You’ll also find the tyre size printed on the sidewall of your existing tyres.

Tip: The tyre size listed in your Certificate of Conformity (CoC) shows the approved and alternative sizes suitable for your vehicle. If necessary, you can request this certificate from your car manufacturer.

Two Essential Rules for Tyre Replacement

1. Always match your tyres to the same size, load index, and speed rating specified by your vehicle or tyre manufacturer.

2. Never install tyres with a smaller size or lower load-carrying capacity than originally approved.

Fitting tyres that don’t meet manufacturer specifications can compromise safety, cause handling issues, and even invalidate insurance coverage.

How to Read the Tyre Size on the Sidewall

Let’s take the example 225/45 R18 95H and break it down:

You can find a detailed explanation of all sidewall markings in the Tyre Markings section.

Understanding Load and Speed Indices

Load Index

The load index represents the maximum weight each tyre can safely support when properly inflated. It’s derived from the permissible axle load divided between two tyres.

For example, a load index of 95 corresponds to a load capacity of 690 kg per tyre.

Speed Index

The speed index indicates the maximum speed a tyre can safely sustain under its rated load.

For instance, a tyre with an H speed rating is approved for speeds up to 210 km/h (130 mph).

Always ensure the tyre’s speed rating meets or exceeds your vehicle’s top speed capability.

Choosing the Correct Tyre Size

When selecting new tyres:

• Follow the vehicle manufacturer’s recommendations exactly.

• Ensure the wheel and tyre diameters are correctly matched.

Incorrect sizing can alter:

• Ride height

• Speedometer accuracy

• Vehicle dynamics and acceleration

Example: Installing 17-inch wheels on a car designed for 14-inch wheels will raise the car’s ride height, slightly increase top speed, and cause the speedometer to display inaccurate readings.

Switching to a different tyre size is possible only if it complies with:

• Legal regulations

• Manufacturer recommendations

• Approved clearances

An authorised test centre must document any approved changes in your vehicle papers.

Driving with unapproved tyres can increase accident risk and lead to fines.

Tyre Construction Compatibility

All tyres fitted to a car must have the same internal construction:

• Either radial or bias-ply.
  Mixing both types on the same vehicle is unsafe and illegal (except in an emergency using a spare tyre).

The same principle applies to wheels or rims — always use those approved by the vehicle manufacturer.

Common Tyre Sizes

Passenger car tyre widths generally range from 125 mm to 335 mm, usually increasing in 10 mm steps.
Some of the most common sizes include 175, 185, 195, and 205 tyres.

Refer to your manufacturer’s documentation or tyre retailer for the full compatibility chart.

Replacing Fewer Than Four Tyres

For optimal safety and performance, always replace all four tyres at the same time.
If that isn’t possible, follow these guidelines:

Replacing Two Tyres

• Both new tyres must be identical in size, load index, and speed rating.

• Always fit the new tyres on the rear axle.

  • Newer tyres offer better grip and aquaplaning resistance, which helps maintain stability in wet conditions.

Replacing One Tyre

• Avoid replacing a single tyre if possible.

• If unavoidable, pair it with the tyre having the deepest tread depth and fit both to the rear axle.

  • Mismatched tread depths or sizes can stress the suspension and transmission, and accelerate tyre wear.

In Summary

Final Tip

When in doubt, consult your vehicle manufacturer, tyre dealer, or approved test centre. They can confirm which tyre sizes are safe, legal, and certified for your car — ensuring every journey remains smooth, efficient, and secure.