Every tyre carries a string of numbers and letters on its sidewall that looks like a secret code. In reality, tyre sidewall markings tell you everything you need to know to buy the right tyre: its size, its load capacity, how fast it is rated to travel and when it was made. Once you can read them, choosing a replacement becomes far easier and far safer.
Fitting the wrong size or a tyre with an inadequate load or speed rating can affect your handling, your speedometer accuracy, your insurance and even the legality of your car on the road. That is why learning to read these markings is worth a few minutes of your time.
This complete tyre size guide walks through each marking in plain English, so understanding tyre numbers becomes second nature the next time you shop for rubber.
Reading a Tyre Size: An Example
Let us decode a common marking such as 225/45R17 94W. At first glance it looks like a random jumble, but every character has a specific, useful meaning. Once you understand the pattern in this one example, you can read the sidewall of almost any passenger tyre on the road. We will take it piece by piece below.
Tyre Width (the first number)
The first three-digit number, 225 in our example, is the tyre’s width in millimetres from one sidewall to the other. A larger number means a wider tyre and a bigger contact patch with the road.
Aspect Ratio or Profile (the second number)
The number after the slash, 45 here, is the aspect ratio. It is the height of the sidewall as a percentage of the width. So 45 means the sidewall height is 45 percent of 225mm. A lower number means a lower-profile, sportier tyre; a higher number means a taller, more comfortable sidewall.
Construction and Rim Diameter (R17)
The letter R means the tyre has radial construction, which is standard on virtually all modern passenger tyres. The number after it, 17, is the wheel rim diameter in inches that the tyre is designed to fit. This must match your wheel exactly.
Load Index and Speed Rating (94W)
The final cluster combines two crucial safety figures. The number, 94, is the load index, indicating the maximum weight each tyre can safely carry. The letter, W, is the speed rating, showing the maximum speed the tyre is certified for. Always fit tyres that meet or exceed your vehicle’s original load index and speed rating.
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Other Important Sidewall Markings
Beyond the core size, your sidewall holds more useful information:
• DOT date code: a four-digit number showing the week and year the tyre was made, which tells you its age.
• Tread wear, traction and temperature ratings: a guide to durability, wet grip and heat resistance.
• M+S or a snowflake symbol: indicating mud, snow or all-season capability.
• Maximum pressure and load: the tyre’s limits, not your everyday recommended pressure.
• Rotation arrows: on directional tyres, showing which way the tyre must spin.
Why Getting These Markings Right Matters
Fitting a tyre that does not match your vehicle’s specification is not just a technicality. A lower load index than your car requires can cause the tyre to overheat and fail under load. A lower speed rating may be illegal and unsafe for your vehicle. The wrong overall diameter throws out your speedometer and odometer, and can confuse safety systems such as ABS and stability control. Matching the markings to your placard keeps everything working as the manufacturer intended.
It is also worth checking the date code on any tyre you are about to buy. A tyre that has sat in storage for years is older than it looks, and you want fresh rubber for the money you are spending.
Once you know your exact size, you can shop with confidence. Browse our full range of car tyres by size, and when you are ready, our mobile tyre fitting team will fit them at your home or workplace anywhere in Sydney.
Frequently Asked Questions
How do I read my tyre sidewall markings?
Read them left to right: the first number is the width in millimetres, the number after the slash is the profile percentage, the letter and number give construction and rim size, and the final figures are the load index and speed rating.
What does the number on a tyre like 225/45R17 mean?
225 is the width in millimetres, 45 is the sidewall height as a percentage of the width, R means radial construction, and 17 is the wheel diameter in inches that the tyre fits.
What is the load index and speed rating?
The load index is a number showing the maximum weight each tyre can carry, and the speed rating is a letter showing the maximum certified speed. New tyres should meet or exceed your car’s original ratings.
Where do I find the right tyre size for my car?
Check the existing tyre sidewall, the placard inside the driver’s door jamb, or your owner’s manual. These tell you the size, load index and speed rating your vehicle is designed to use.
Can I fit a different tyre size to my car?
Minor variations are sometimes possible, but changing size affects speedometer accuracy, handling and safety, and may not be legal. It is best to stick to the manufacturer’s specification or seek expert advice before changing.