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How to read your tyre size

There is no denying that understanding the jargon that is used by the mechanic trade can be confusing at times. Having an understanding of some of the terms used gives you more self-confidence and helps you choose the right tyre fitment company. Here's everything you need to know about tyres and how it can help you.

Importance of knowing how to read your car’s tyre size

The ability to read and understand your car's tyre size is invaluable, especially when you are shopping for new ones. Having the right tyres on your car is crucial for safety and performance. Thus, you need to choose tyres based on the conditions you drive in and how your vehicle handles. To make the best decision, you need to know how to read tyre sizes.

You might not have noticed the numbers and letters on your tyre before, but they're packed with useful info. Not only can you tell the tyre size by these jumbled numbers and letters, but you can also know how safe or old they are.

Where to find your tyre size

Usually, a tyre's size is marked on its sidewall, which is the smooth vertical strip between the tread edge and the bead.

When you check the size of your tyre, you'll see that it's composed of letters and numbers. It can be tricky to figure out your tyre size at first, but with some practice it becomes easier.

The size of your tyres can also be found in the owner's manual of your vehicle. This can be found on the dashboard, inside the glove compartment, or on the door panel of the driver's door.

How do you read your tyre size?

The sidewall of each tyre is usually marked with numbers and letters as follows: 195/55R/16/91H. These numbers and letters all represent something different, providing valuable information about the size, construction and type of tyre.

195 A tyre's width in millimetres

55 Sidewall height divided by the width of the tyre

R Indicates the tyre's construction

16 Diameter of the rim in inches

91 Load index

H Speed rating

Tyre width

The tyre width is the distance between a tyre’s inner and outer sidewalls (in millimetres). This is indicated by the first three digits on the sidewall. In our example, the first set of numbers means that the tyre measures 195mm in width. As another example, if your tyre reads 235/45R19, then the tyre width is 235mm.

Tyres for passenger cars usually have a letter P on the tyre width. For industrial and heavy-duty tyres, you might see LT instead, referring to light-truck tyres. In addition, there can be letters T, indicating temporary spares, or ST, indicating special trailers.

Aspect ratio

After the tyre width and the first slash, comes the aspect ratio. You can think of it as the height of a tyre compared to its width. Using the example above, 55 is the aspect ratio of the tyre. This means the tyre sidewall’s profile height corresponds to about 55% of its total width, or 107.25mm. When the aspect ratio is larger, you can also expect the sidewalls to be thicker.

Tyre construction type

Following the aspect ratio, you'll find a letter. As shown in the example, we see R. The R refers to Radial, which describes a specific tyre construction method. With a Radial tyre type, the inner steel belt angles 90 degrees towards the travelling direction, increasing the tyre’s strength. The vast majority of modern car tyres are radials. Sometimes, alternative letters are used to identify tyres with different constructions. For instance, B and D may be used to refer to bias ply or diagonal construction.

Rim diameter

After the tyre construction type and slash, the wheel's diameter in inches, also known as the rim diameter, follows. It measures the width of the wheel across the centre. It’s how far apart the two benches are. According to our example, the rim diameter is 16.

Load index

Next is the load index, represented by the two digits after the rim diameter. The load index measures the maximum load a tyre can handle at maximum speed if it is inflated correctly. The two-digit numbers represent different weights in kilograms. In the example above, 91 is the load index, indicating a weight of 615kg. Knowing your tyre’s load index is essential, as it keeps you from overloading your vehicle, which could be dangerous. Overloading increases fuel consumption, affects handling and sometimes results in serious tyre damage.

Speed rating

The final letter indicates the tyre's speed rating, which is the speed it can travel when loaded to its maximum capacity. Using our example, the speed rating is H, which translates to 210kmh.

Other details you can find on the sidewall

A tyre's sidewall also contains other information, such as the manufacturer’s brand, the maximum inflation pressure and its model. It may also contain the following:

Tyre age

Each tyre has a three-digit age code that indicates when it was manufactured. Since the quality of tyres degrades over time, especially with infrequent use, it’s advisable to check the condition of your tyres if they are older than six years.

Reinforced Tyres

A tyre with REF indicates that it is built to handle the additional weight. Manufacturers may have different specifications, so look for EXL and XL for extra load and REINF, RFD and RF for "Reinforced".

Run-flat Tyres

Also, some types of tyres on the market today are built to continue running for a limited time after they have been punctured or lost pressure. Tyres with reinforced walls are designed to bear the vehicle’s weight for a brief period, at least until the vehicle reaches a safe location. For running flat tyres, look for indicators, such as ROF and EMT.

Snow condition Tyres

Lastly, tyres with M+S (mud and snow) or a logo featuring a three-peaked mountain with a snowflake are designed to provide extra traction during winter conditions.

Imperial Tyres offers high-quality tyre services in Swindon

By understanding tyre sizes, shopping for tyres can become much easier. If you’re looking for a ‘replacement tyre near me’ or you require other quality tyre services, Imperial Tyres is here to assist you. Contact our tyre specialists today.