A Complete Guide to Understanding Car and Motorcycle Tyre Numbers

A Complete Guide to Understanding Car and Motorcycle Tyre Numbers

When we look at the sidewalls of our vehicles' tyres, we are likely to notice a series of numbers and letters that can seem cryptic. However, each number and letter serves a crucial purpose, providing detailed information about the tyre's specifications and characteristics. Understanding this nomenclature is vital to choosing the right tyres for your car or motorbike, ensuring safety and performance.

In this article, we explain in detail what these numbers mean, so you can make informed decisions when buying new tyres or simply better understand the ones you already have.

What do the numbers on tyres mean?

A typical code you might find on a tyre is something like: 205/55 R16 91V. Here's a breakdown of what each part of this code means:

Tyre width (205)

The first number in the sequence, in this case 205, indicates the width of the tyre in millimetres. This value is measured from the outer (side) sidewall to the inner sidewall of the tyre. In the example, the tyre has a width of 205 millimetres.

A wider tyre can provide more grip on the road, which is beneficial in terms of stability and handling. However, it can also increase fuel consumption due to higher rolling resistance.

Aspect ratio (55)

The second number you see, in this case 55, is the aspect ratio, which represents the tyre's sidewall height as a percentage of its width. In this example, the sidewall height is 55% of the tyre width, i.e. 55% of 205 millimetres.

A lower aspect ratio generally means a sportier tyre with better handling, but it can also result in a stiffer ride. Tyres with higher aspect ratios, on the other hand, provide a smoother ride but may be less agile in corners.

3. Tyre construction (R)

The letter R indicates that the tyre has a radial construction, the most common type in modern tyres. This means that the fabric plies of the tyre are arranged radially, perpendicular to the direction of travel. Radial tyres offer better stability and longer life compared to tyres of diagonal construction (indicated with a ‘D’).

4. Rim diameter (16)

The next number, in this case 16, refers to the rim diameter in inches. This means that the tyre is designed to fit on a 16 inch diameter rim.

It is essential to choose a tyre with the correct diameter for your vehicle's rim, as a mistake in this respect can affect handling and, in some cases, may be impossible to fit.

5. Load index (91)

The load index is a number that indicates the maximum load capacity the tyre can carry. In this example, 91 corresponds to a load capacity of 615 kg. Each number in the load index table has a corresponding weight, which is important to ensure that the tyres can carry the full weight of the vehicle when loaded.

If you fit tyres with too low a load index, you could put safety at risk, as the tyres could deform or fail under excessive weight.

6. Speed index (V)

The last letter, in this case V, indicates the speed index, which defines the maximum speed the tyre can safely withstand. The V-index means that the tyre is designed for speeds up to 240 km/h (150 mph). Other common indices are ‘H’ (210 km/h), ‘T’ (190 km/h) and ‘W’ (270 km/h).

It is essential to use tyres with an appropriate speed rating for your vehicle, especially if you regularly drive at high speeds. Failure to do so can increase the risk of accidents due to tyre overheating.

Other Tyre Markings

In addition to these main codes, tyres also have other important markings that are worth knowing:

DOT (Department of Transportation): This code indicates that the tyre meets U.S. Department of Transportation standards and is usually accompanied by a date code indicating when the tyre was manufactured (e.g. ‘4223’ means week 42 of the year 2023).

TWI (Tread Wear Indicator): This is a wear indicator that helps you know when it is time to change the tyre. These are small bars between the main grooves of the tyre; when the tread is at the same level as these bars, it is time to replace the tyre.

M+S (Mud and Snow): This symbol indicates that the tyre is designed to perform well in mud and light snow conditions. However, it does not always guarantee good performance in extreme winter conditions; for that, look for the symbol of a snowflake with a mountain.

Rotation: Some tyres have an arrow indicating the direction in which they should be rotated for optimum performance.

Why is this information important to know?

Choosing the right tyre for your car or motorbike is crucial for several reasons:

Safety: Wrong tyres can affect vehicle stability and increase the risk of accidents.

Performance: The right tyres ensure better handling, braking and grip in different weather conditions.

Economy: Fitting the right tyres can improve fuel consumption, reduce wear on other components and extend the life of the tyres themselves.

The numbers and letters on the tyres provide all the key information you need to ensure your car or motorbike is using the right tyres. From width, profile height and rim diameter to load and speed ratings, every aspect of these codes is designed to give you optimum safety and performance on the road.

By having a clear understanding of what each code means, you can make more informed decisions when replacing your vehicle's tyres. Not only will this improve your driving experience, but it can also help you avoid accidents and maintain the efficiency of your car or motorbike.