Looking for replacement tyres starts with a few numbers and letters on the sidewall. A marking such as 225/40R18 tells you the tyre’s width, profile, construction and rim diameter. The number and letter that follow it describe its load index and speed rating. Understanding each part makes it easier to narrow your search and ask the right questions before buying.
Start with your vehicle’s tyre placard
Your current tyre’s sidewall is a useful starting point, but check it against your vehicle’s tyre placard and owner’s manual. The placard is commonly inside the driver’s door opening, although its location varies between vehicles.
These references identify the manufacturer’s recommended tyre specifications. Checking them is particularly useful if you bought the vehicle second-hand or its wheels have been changed. Copying the tyres already fitted could repeat an earlier sizing mistake.
What does 225/40R18 mean?
225 is the width. It gives the tyre’s nominal section width in millimetres, measured between the sidewalls. In this example, the tyre is nominally 225 mm wide.
40 is the profile, also called the aspect ratio. It expresses the sidewall height as a percentage of the tyre’s width. Here, the nominal sidewall height is 40% of 225 mm, or about 90 mm. The number is a percentage rather than a measurement in millimetres.
R means radial construction. It describes how the tyre is built internally.
18 is the rim diameter in inches. This tyre is designed for an 18-inch rim. That measurement describes the rim diameter rather than the overall height of the tyre and wheel together.
Read all four parts when searching. Two tyres designed for an 18-inch rim can still have different widths, sidewall heights and overall dimensions.
Load index and speed rating have different jobs
After the size, you may see a marking such as 92Y. The number is the load index and the letter is the speed rating.
The load index is a code linked to the maximum load a single tyre can carry under its specified operating conditions. The number itself is not a weight in kilograms. A load index chart translates it into a load capacity.
The speed rating identifies the tyre’s speed capability under specified conditions. Tyre condition, correct fitting and inflation also matter.
Tyres with identical size markings can have different load indices and speed ratings. Confirm both against the vehicle manufacturer’s requirements when comparing replacements. A higher tyre load rating also does not increase the vehicle’s own carrying capacity.
Check the front and rear tyres
Some vehicles use different tyre sizes at the front and rear. Check the markings on both axles and compare them with the placard or owner’s manual.
Different sizes may be part of the original specification. They may also need investigation if they do not match the vehicle’s requirements. Avoid assuming one tyre’s marking applies to all four wheels.
If you are considering a different wheel or tyre size, get the complete fitment checked first. Rim diameter alone cannot confirm suitability; dimensions, clearance and load and speed requirements also need consideration.
Find your size, then confirm your choice
Once you have confirmed the specification, enter the width, profile and rim diameter in the Tyreforce tyre finder. Keep the required load index and speed rating handy when comparing options.
For help checking your tyre size or discussing replacement tyres in Campbellfield, contact Tyreforce with your vehicle details and photos of the sidewall markings and tyre placard.
Further reading: Michelin tyre markings · Michelin load index and speed rating · Bridgestone tyre sizes · Michelin replacement tyre sizes.
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