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How to Read Tyre Sizes

How to Read Tyre Sizes

How to Read Tyre Sizes? Reading and understanding passenger car tyre sizes may seem daunting at first, but it’s actually quite simple once you know what all the numbers and letters mean.

In this blog post, we’ll take you through everything you need to know about reading car tyre sizes so that you can make an informed decision next time you’re shopping for tyres.

How to Read Tyre Sizes.

The first thing you need to know is where you’ll find sizes of front and rear tyres.

These might be printed in the owner’s manual – but it’s better to read them from the tyres directly. This is because different wheels or tyres could be fitted – either by the factory, the dealership, or previous owners.

You’ll find the tyre size moulded into the tyre’s sidewall markings – that’s the bit of the tyre that faces outwards – pretty much flat facing you as you kneel down to look.

How to Read Tyre Sizes
275/45 R21 Sized tyres, You can see the number on the side wall.

Different size formats Metric and Imperial.

There are two types of car tyre size systems in use today- metric and imperial. We’ll touch briefly on imperial sizes – but around 99% of tyres sold in Australia are measured and marked up in metric sizes, so these will be the main focus of this guide.

Once you’ve determined which system your car uses, it’s a matter of understanding what all the numbers and letters on the tyre represent.

Metric sizes

How to Read Tyre Sizes that are Metric

For metric tyres, there will be a sequence of three numbers followed by two letters. The first number is the tyre width measured in millimetres; the second number is the height of the sidewall as a percentage of the width (sometimes called the ‘aspect ratio’); and the third number is the diameter of the wheel rim in inches. The letters at the end indicate the type of tyre- for example, ‘P’ means passenger vehicle and ‘C’ means commercial vehicle.

Example metric tyre size: 205/55R16 91V

  •  Width = 205 mm
  •  Height = 55% of 205 mm = 113 mm
  •  Tyre Diameter = 16 inches or 406 mm
  •  Type = Passenger vehicle tyre with a 91 load index (maximum load capacity) and 220 km/h maximum speed rating

You’ll also notice that there’s an ‘R’ printed in the middle of those numbers. This refers to the tyre construction method. In this case, R means ‘radial ply construction’. This is a technical term and virtually all road-going car tyres will be marked R. If you need anything different, your retailer will let you know.

Imperial sizes

Some 4×4 and  imported cars may have tyres with imperial measurements.

How to Read Tyre Sizes that are imperial.

They can have either two sets of numbers or three on the tyre sidewall. They’ll look like this:

Example imperial tyre size 1: 8.5-R18

  •  Tyre Width = 8.5 inches
  •  Diameter = 18 inches

Example imperial tyre size 2: 18×8.5-6

  •  Width = 8.5 inches
  •  Diameter = 18 inches
  •  Height = 6 inches

Need help with imperial tyres sizes? Don’t worry – when you request a quote through Auto Hero, the tyre retailers we put you in touch with will be able to convert these tyre sizes with a tyre size calculator and explain which metric tyres will fit.

What about tyre pressure figures?

You’ll usually also find other tyre specifications figures printed on the sidewall of the tyre. Sometimes, these will be model numbers and will be printed in large figures. Often though, you’ll see smaller numbers – for examples 29-36.

This indicates the optimum tyre pressure that the tyres should be inflated to. We’ll explore this elsewhere – but these figures don’t relate to actual tyre size.

Specialist markings on run-flat tyres

Run Flat Tyre

Some tyre manufacturers have products known as ‘Run-flat tyres’.

These are designed with a tough sidewall that won’t flatten even if you loose pressure. This means there’s no need for swapping to a spare wheel or trying a roadside repair.

You might be asked if your tyres are ‘run-flats’ as these are sometimes added as standard by a vehicle manufacturer. If so, you’ll see see different markings depending on the brand of tyre on your car.

Goodyear, Bridgestone, Dunlop, and others

These tyres use an ‘ROF’ marking on their sidewall. This stands for ‘Run on Flat’.

Some other Goodyear tyres

Some Goodyear tyres use the term ‘EMT’ – meaning ‘Extended Mobility Technology’.

Michelin, Yokohama, and others

These brands use either ‘ZP’ or ZPS’ – an in-house designation used to identify run-flats.

Understanding your current tyres

Now that you knowHow to Read Tyre Sizes, you’ll be able to make an informed decision next time you need to buy tyres for your car.

If you have any further questions about car tyres, the tyre retailers we put you in touch with when you request a quote will be happy to help! They’ll answer questions about correct tyre size, load carrying capacity, and much more!

Still Need Help?

Don’t worry if you haven’t quite worked it out – lots of workshops will even just recommend that you send a quick picture of the tyre if you’re not sure.

When you look for new tyres through Auto Hero, we promise you’ll get excellent customer service – whether you can work out your tyre size or not!

Our tyre specialists will walk you through everything you need to know.

Click here to get a tyre quotes from Helpful store near you.


Written By

Matt Banks

Matt is the founder of Autohero.com.au and has been hooked on cars and repairs ever since childhood. A veteran in automotive since leaving school, Matt has completed his trade as a panel beater and is across all things with wheels. 

His first car was a 1967 FORD XR Wagon followed by a string of collectable Holden’s.

Have a question for Matt? leave a comment below.

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