Car tyres do a magnificent job of keeping your vehicle on the road in a huge range of conditions, but to perform in the best way, and to keep you safe, it is important that you have the right sized tyres for your car. Tyre sizes come in a bewildering variety and it can be easy to pick something that isn’t right for your car. 

In order to get the best tyres that you can, you need to understand the tyre markings that are displayed on your car tyre and establish if what you have fitted are the most appropriate for your car.

While you should always seek professional assistance in selecting and fitting car tyres, it is very helpful to understand the figures that are displayed on them.

Where do I find the information?

Typically, the information regarding your tyre sizes is moulded into the tyre sidewall, and is displayed as a sequence of numbers and figures around the periphery of your tyre. You may need to kneel down next to your tyre to find the start of the information – which is usually in quite large letters and numbers – and then read it round.

The same number and letter sequences can usually also be found in your car’s user manual, which is written by the manufacturer and will always be the right information.

What does the sequence mean?

The sequence is split up into several distinct parts, each giving a specific piece of information about the tyre and its features. The sequence breaks down as:

  • Section width: The width of the tyre in millimetres.
  • Aspect ratio or Profile: The ratio of the tyre's width to height, as a percentage.    
  • Rim diameter: The diameter of the rim which the tyre is to be fitted to, in inches.
  • Load index: Indicates the maximum load that each tyre can support.

There is also a prominent letter displayed, which indicates the speed rating that the tyre is designed for. These range from Q, which denotes a maximum speed rating of 100mph, up to Y, which indicates a maximum speed rating of 186mph. In addition, the letter may be contained within brackets, to indicate a higher speed. In this way, (Y) indicates a maximum speed of over 186mph.

With respect to the aspect ratio, a so-called low profile tyre has shorter side walls which aren't as flexible as regular profile tyres and allow greater road-holding because there is less ‘flex’ in the sidewall. In this way, shorter side walls increase your car's road holding, grip around corners and are considered to improve the look of your car.

However, low profile tyres are usually more expensive than normal tyres and are less comfortable to drive on as they pick up more bumps and undulations in the road.

The sequence of numbers and letters are always the same font and form, and usually stand out from any other writing on the side of the tyre.

For example, you might find the following around your tyre – 185 60 R 14 82 H

This would mean:

  • 185 – the tyre section width in millimetres
  • 60 – the aspect ratio in % (the height of the sidewall divided by the width of the tyre)
  • R – Denotes the tyre's construction type - in this case it's a radial tyre
  • 14 – Rim diameter in inches
  • 82 – Load Index 
  • H – Speed Rating

This information tells us that the tyre has a section width of 185mm, and an aspect ratio of 60%, it is a radial design and for a 14-inch wheel. The load index is 82, and maximum speed rating is 130 mph.

Choosing the right tyre size

While you should always choose the tyre size that is appropriate to your car, some of these factors can change, particularly if you make modifications to your car, as many are fond of doing.

The first factor is the load ratio - the maximum weight the tyre can handle at a time. Making changes to the mass of the car, either by reducing it to increase speed and manoeuvrability, or increasing its mass by fitting a larger engine or running gear, can change the ratio. In this case, you will need to evaluate the change in mass and select tyres that are appropriate.

You should also consider tyre width. The tyre width can be changed easily, even if the other factors are left the same. Changing the tyre width can make a car look a lot sportier, and can make it more stable on the road, but you have to be careful with your choice. If you go too far either way – narrow or wide – you may upset the dynamics of the vehicle or end up with the tyre rubbing dangerously against the inside of the wheel arch.

The diameter of the tyre is another factor worth considering. Sometimes people change the diameter of their wheels for cosmetic purposes, but this can lead to problems with both speedometer and odometer readings as the circumference is now different.

Your car comes with factory-installed tyres that are 550 mm in diameter. That means the circumference of each tyre is 1,740 mm. Now let’s say you want to replace the standard tyres with non-standard tyres that are 625 mm in diameter. Each new tyre has a circumference of 1,964 mm, which means it travels 224mm farther with each complete revolution. This has a tremendous effect on your speedometer, which will now indicate a speed that is too slow by almost 13% compared with the standard tyres. 

So, when your speedometer reads 60 mph, your car will actually be traveling 67.7 mph! It goes without saying that if you went for smaller tyres, you would be going faster than your speedometer indicated, putting you in a potentially illegal position.

Of course, your odometer reading would also be different, so if you want your car’s odometer and speedometer to be accurate, always select the right tyre size for your car.

Why is the external diameter an important parameter?

The external parameter is very important when buying aftermarket or replacement products.

If you ever change your tyre or rim size, the resulting product should still have the same external diameter as the original tyres. As a rule, the difference should not be smaller than 2% of the original diameter, or larger by 1.5%.

The standardised sizes of tyres and rims 

Tyres have been standardised so as to make the choice easier for suppliers to stock and for customers to understand. A standardised system makes it easy to convey information regarding your needs to a supplier over the phone or internet. Standardisation also:

  • Limits the number of tyres that a supplier has to hold as stock.
  • Ensures that the entire industry works to a common database.
  • Prevents tyre manufacturers from creating bespoke systems.
  • Makes tyres easier to replace across manufacturers and brands.

Why are tyre profiles expressed as a percentage?

The tyre profile – essentially the height of the sidewall – is given as a percentage so that it is possible to establish the shape of the tyre as a relationship between the height and width of the tyre. As profiles have changed over the years from the 1020’s onward, with each decade seeing an increase in profile to aid road holding.

Because of the changing tyre size, showing the profile as a percentage is the most practical way to display the information. Using this system even a small change in increment can be used to describe tyres of different types, dividing the whole tyre range into a dozen or so size groups in a convenient way.

With this system, the higher the aspect ratio of a tyre, the taller the sidewall of a tyre will be. Heavy and large vehicles like buses and trucks have tyres with aspect ratios as high as 95. Tyres with high aspect ratio are usually found on slower vehicles and may be less good on handling but provide a more comfortable ride by cushioning bumps due to more air in the tyre.

Conversely, high performance and racing cars have tyres with a low aspect ratio. Low profile tyres with shorter and stiffer tyre walls that allow the tyre to resist fast cornering forces better, improving handling traits of a car by reducing roll as it corners.

Why is the rim size given in inches?

If the width is expressed in millimetres, it is only natural that drivers question why the rim size of the wheel is given in inches. Rim sizes have always been given in inches, as this keeps the numbers consistent and fits with the established system. Changing the rim size to a metric system would mean the numbers were inconsistent. The same would also happen changing from a metric to imperial tyre size, so to ensure the numbers are in an orderly fashion, it is necessary to use two different systems.

Tyre size systems

There are two compatible tyre sizing systems currently in use in the world:

  • The European metric system
  • The American p-metric system

The Eurometric system

This is a common system that is used all over the world. A typical tyre size is written like this: 195/65 R15 91T. There are, however, still a few metric-system tyres with profiles of 80 and 82. These so called “full-profile” tyres typically omit the profile altogether, such as “155 R13” tyres. In this example, these would be equivalent to 155/80 R13 tyres.

Furthermore, if a Eurometric tyre has a “C” at the end, it means the product is designed for a commercial vehicle. So, a 195/70 R15 C tyre is, in essence, the same as a 195/70 R15, apart from its intended purpose and the vehicle type it is used on.

American tyre size comparison

The North American P-metric system is the main alternative to marking tyre sizes. It was first introduced by American tyre manufacturers in 1977 and typically looks like this: P205/80 R14 93 S. If the size begins with a “P”, it marks the tyre as fit for passenger cars, as well as minivans, SUVs and small pickup trucks. The figures “LT”, on the other hand, signifies products for light trucks.

Other than this, this system appears similar to the European system, so the equivalent to a European “C” may be an LT 245/75 R16 108/104S sized tyre.

Imperial tyre sizes

The “imperial sizing” was commonly used before and today, this is mostly used for historical vehicles and vintage car tyres. Under this system, sizes were either given in inches or alphanumerically.

For the former, such as 6x13, the first number was the width, while the second referred to the rim diameter. Both numbers were in inches and any profile information was given. If you have a vintage car, you will need to buy tyres using this system.

Additional tyre size systems

Spare or emergency wheels have a T in their marking, labelling them as a temporary spare. So, a T125/90 D16 98M tyre is specifically designed as a temporary product and should not be driven on for extended periods of time. Replacement tyres have a much thinner section, making them easy to store in the car without taking up too much room, and are usually inflated to very high pressures to support the car on such a thin section.

Professional rally tyres, like those used in motorsport, have different markings depending on the manufacturer, and are different again from those found in normal tyre systems. The rim diameter, for example, is sometimes given next to the width, as this information can be quite relevant during a pit stop change.

If you’re looking to choose an alternative tyre to the original equipment supplied with the vehicle, there are a range of aftermarket tyres available. However, when it comes to ensuring the right diameter, there is a general rule that the external diameter should not exceed the original tyre size by more than 1.5% and it should not be smaller than 2%.