Tyre Sizes Explained: How To Read Your Tyre Sidewall Correctly

Your tyre sidewall tells you its size, construction, load capacity, speed rating and other important specifications. Learning how to read these markings makes it easier to buy a suitable replacement and spot important tyre information. If you're unsure, a tyre shop on the Central Coast can confirm the correct specification for your vehicle.

Why Your Tyre Sidewall Matters

The letters and numbers moulded into a tyre aren't random. They identify important characteristics that affect whether the tyre is suitable for a particular vehicle and application.

Understanding them is useful when replacing worn tyres, comparing products or buying used tyres. It also helps you avoid choosing a tyre simply because it appears to be roughly the same physical size.

Your vehicle manufacturer specifies suitable tyres based on factors such as dimensions, load capacity and performance requirements. You can usually find tyre information on the vehicle's tyre placard, often located around a door opening, glovebox or another manufacturer-specified location.

Use that information alongside the tyre sidewall when you're looking for replacements.

How To Read Your Tyre Size

One of the most prominent markings will look something like:

225/60 R17 99H

Each section tells you something different. Using this example:

• 225 is the nominal tyre section width in millimetres.
• 60 is the aspect ratio. The sidewall height is 60 per cent of the tyre's section width.
• R indicates radial construction.
• 17 is the wheel rim diameter in inches.
• 99 is the load index.
• H is the speed rating.

A common mistake is assuming the second number represents a measurement in millimetres. It doesn't. It's a percentage of the tyre's width. Changing one part of the tyre size can affect more than appearance. Overall diameter, clearance, load capacity and compatibility with the wheel and vehicle all need consideration.

Don't change tyre sizes simply because another option physically fits the rim. Check that the proposed tyre is suitable and compliant for your vehicle.

Once you've identified the size, Budget Tyre Shed's guide on how to choose the right tyres for your vehicle can help you consider the next step.

Understanding Load Index And Speed Rating

The final numbers and letters in a typical tyre size are easy to overlook, but they're important.

What Is A Tyre Load Index?

The load index is a numerical code corresponding to the maximum load the tyre can carry under specified conditions. In our example, 99 is the load index. It isn't a direct weight measurement, so you need a load-index table to determine the corresponding load capacity.

Replacement tyres need to meet the applicable requirements for your vehicle. This becomes especially important for utes, SUVs, vans and vehicles that regularly carry significant loads.

What Is A Tyre Speed Rating?

The letter following the load index indicates the tyre's speed category under specified conditions. Different letters correspond to different speed capabilities. However, that doesn't mean the rating is a recommended driving speed or permission to exceed Australian speed limits.

Load and speed ratings should be checked as part of the complete tyre specification rather than choosing replacements based only on width and rim diameter.

Finding Your Tyre's Manufacturing Date

Tyres also carry manufacturing information. You may see a DOT marking on tyres manufactured to relevant US Department of Transportation requirements. At the end of the relevant code, a four-digit number can identify the tyre's manufacturing week and year. For example:

2425

This indicates manufacture in the 24th week of 2025. This information can be particularly useful when assessing a used tyre or checking the age of a tyre that has spent a long time on a vehicle.

Age alone doesn't tell you whether a tyre is safe. Storage conditions, use, damage, deterioration and overall condition also matter. Check for signs such as cracking, deformation or other deterioration, particularly on vehicles that cover relatively few kilometres and may retain the same tyres for long periods.

Treadwear Indicators And Legal Tread Depth

Tyres have treadwear indicators moulded into their main grooves. These raised sections help show when tread has worn down to its minimum level. In Australia, the commonly applicable minimum tread depth for passenger vehicle tyres is 1.5 mm across the required tread area.

However, waiting until tyres reach the legal minimum isn't always the best approach, particularly if they show damage, uneven wear or deteriorating wet-weather performance. Check tread at several points around each tyre rather than inspecting one convenient spot. Uneven wear can indicate other problems.

For example, excessive wear on one shoulder or across particular sections may warrant investigation of tyre pressures, suspension or wheel geometry. If treadwear indicators are appearing unevenly, a wheel alignment may be worth checking alongside the condition of the tyres.

Never rely solely on tread depth if a tyre has cuts, bulges, exposed cords or other obvious damage. Have it professionally inspected.

Safety Standards And Compliance Markings

A tyre sidewall can contain many additional codes and markings depending on the manufacturer, tyre type and markets for which it was produced. You may see information relating to:

• Tyre construction
• Maximum load
• Inflation limits
• Manufacturer identification
• Manufacturing codes
• Directional rotation
• Inside and outside mounting orientation
• Reinforced or extra-load construction
• Tubeless construction
• Homologation or compliance markings

Some performance tyres are directional and need to rotate in a specified direction. Others are asymmetric and have a designated inside and outside face.



These markings matter during fitting. Mounting a tyre incorrectly can prevent it from performing as its manufacturer intended. If you're not sure what a code means, don't guess based on a similar tyre. Check the manufacturer's information or ask a tyre professional.

Using Sidewall Markings When Buying New Or Used Tyres

Knowing how to read tyre sidewall markings makes comparing replacement tyres considerably easier. Start with the vehicle's required specifications rather than simply copying one number from the existing tyre.

This is especially important if you've purchased a used vehicle, because the tyres currently fitted may not necessarily be the original specification. When buying a new tyre, check:

1. Tyre dimensions
2. Load index
3. Speed rating
4. Wheel compatibility
5. Tyre type and intended use
6. Any directional or mounting requirements

Used tyres require additional checks. Inspect the remaining tread, wear pattern, age and overall condition. Look closely for cuts, cracking, bulges, repairs or other damage. A tyre can have plenty of visible tread remaining while still having other issues that make it unsuitable.

Example: A Central Coast driver needs to replace a damaged 225/60 R17 tyre and finds a used tyre with the same three size figures. Before buying it, they also check the load and speed ratings, manufacturing information and physical condition. Matching the visible dimensions alone wouldn't provide enough information to make a suitable replacement choice.

Whether you're buying new or quality used tyres on the Central Coast, bringing your vehicle details or existing tyre specification can make the process much quicker.

Get Expert Advice From Your Local Tyre Shop

Tyre sidewall markings give you useful information, but they don't replace checking what is appropriate for the vehicle. Your tyre placard, manufacturer specifications, vehicle use and applicable requirements should guide replacement decisions. If the vehicle has non-standard wheels, suspension modifications or a different tyre size from factory, getting professional advice becomes even more important.

The key takeaway is to read the whole tyre specification, not just the width and rim diameter. Size, load index, speed rating, condition and vehicle compatibility all matter. If the markings still have you scratching your head, Budget Tyre Shed is your local tyre shop on the Central Coast to help identify suitable new or used replacement tyres. Bring your vehicle in and the team can check what you're currently running and discuss your options.

Frequently Asked Questions