What do the numbers on the side of tyres mean?

Tyres are sized by width, diameter, and aspect ratio, but what do these numbers even mean? How do I know if I'm buying the right tyres?

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Despite most tyre-fitting shops having a crosschecking catalogue, it's always a good idea to know what each number means in case you want to branch out from your factory tyre size.

An example of a tyre size is 175/60 R13 77T. If that sounds like a bunch of mumbled letters and numbers together, don't worry – by the end of this, you'll have no trouble deciphering it. Let's start with the first numbers and work our way down the line.

Width
The first set of numbers is the tyre's width. So the '175' on a 175/60 R13 77T refers to the tyre being 175mm wide. These numbers usually go up in 10s, so the next tyre size up would be 185, and the next size down would be 165.

Profile
The profile is measured in a percentage of the tyre width. The '60' in 175/60 refers to the sidewall height being 60 per cent of 175mm, meaning the sidewall height is 105mm.

Sidewall heights between different brands are often not the same. A 175/60 cheaper non-branded tyre will often look slightly lower-profile than a 175/60 from major brands like Bridgestone, Michelin, Pirelli, etc.

Construction
The vast majority of tyres on the market are radial, meaning they are made of a combination of rubber and steel belts. This is where the 'R' comes from.

There is also B for 'Bias' tyres, but cars have not used these since the 1970s.

And then there is S for 'Solid'. You find these on heavy equipment or your wheelbarrow.

Diameter
This one is simple and keeps with the trend of using a different unit of measurement for each dimension. The '13' in 175/60 R13 means it goes onto a 13-inch diameter wheel rim.

Load rating
This number has to be crosschecked with an index; it tells you how much weight each tyre is rated for. The '77' in 175/60 R13 77T would mean each tyre is rated for 412kg. The load rating index is the same around the world.

As per what is written in 24A of the Australian Design Rules for Tyre Selection: "The tyres fitted to this vehicle shall have a maximum load rating not less than XXkg, or a load index of XX and a speed category of X". Meaning it is illegal to have tyres with a lower load rating than what is stated for your vehicle.

Load rating examples:

Speed rating
The final character in tyre sizing is a letter that signifies the maximum speed the tyre is rated to travel.

Following the same rules as load ratings, you must fit a tyre with the same or better speed rating than what comes stock on the car.

Using the index below, the 'T' in 175/60 R13 77T means the tyre can travel at 190km/h.

Speed rating index:

All cars come with a tyre-size plaque that is usually found in the front door jamb, under the bonnet, in the glovebox, and sometimes on the fuel door. This will have all the information on the size of your car's tyres from the factory and the required tyre pressures.

Different states have different regulations surrounding differing tyre sizes. Some states will not allow you to stray too far away from the stock tyre size without engineering, so it's always a good idea to check before changing.

Now you're well versed in the language of tyre sizing, you can go out and explore the wide world of different tyres, and may even be able to find a tyre perfectly suited to your driving style by doing your own research!