Decoding Tyre Sizes

By peeling back the layers and taking a bit of a deeper dive into the many tyre sizes and deciphering the codes which you see on the surrounds of your tyre, you’ll be able to comprehensively make an informed decision on the tyre size needed for your car and how certain changes within tyres will hinder or enhance your driving experience.

So, what are tyre sizes? 

Tyre sizes are standardized measurements and coding systems which are used on every vehicle's tyres to decipher the specific dimensions and characteristics of the tyre. The information precisely codes for information about the tyres width, aspect, construction, diameter, load capacity and speed ratings, and conveniently, all of this information can often be found on the surface of the tyre itself.

Decoding Sidewall Displays 

Lets get into how to actually understand the nitty gritty information of what your tyre size is trying to tell you. There are numerous components of letters, numbers, and symbols which are often included on the tyre to demonstrate their size and application. Please remember that this is standardised information and now all tyres are manufactured within the same guidelines so if your tyre does not represent the same components as described here, try to not instantly think that you have a dodgy tyre. The first step is to identify the combination of icons on the side of your tyre. 

Tyre Type Indicator

The first letter which is found on the tyre is the tyre type indicator 

• P - If the letter you find is a “P”, this is conveying the information that they are passenger tyres which are commonly found on cars, minivans, and SUVs. In other words, they are named implying passenger cars as they are often filled with passengers 

• LT - If you find LT, this stands for Light Truck tyres which are designed for use on trucks, SUVs and vans which carry heavy loads or designed for trailer towing 

• ST - Special Trailer is indicated through the letters ST which are, as you guessed, designed for trailers 

• T - When you find T on your tyres, this stands for Temporary which displays that the tyre is specifically and primarily designed for temporary use only and is what you will find on your spare tyre  

Tyre Width - When it comes to measuring the tyre, it is done in the unit of millimetres from each sidewall of the tyre. As an example, if your cars first four icons were P210, it would mean that your Passenger car has tyre sizes of 210 millimetres. 

Aspect Ratio - The aspect ratio conveys the percentage of the height in comparison to the width of the tyre, meaning that, if we built the prior example, a tyre size showing P21070 would convey that the sidewall is 70% of the width of the tyre. 

Construction Type - There are different letters when it comes to this section which all correspond to the different types of construction of the tyre itself.

• R - A R stands for Radial construction which indicates that plies run perpendicular to the direction in which they travel to provide the most optimal traction and durability 

• D - D indicates that the tyre is of Diagonal construction therefore conveying the information of the plies running diagonally across the tyres treads. Diagonal, aka bias treads, treads are most common in older vehicles or those desiring the speciality applications of diagonal plies, such as a smoother ride on tougher terrains 

• B - “Bias Belted” construction is the corresponding meaning behind the display of B on your tyre, thus meaning that these tyres have both the bias and radial ply constructions

Wheel Diameter - Once again continuing the numbers from before, P21070R12 would indicate that the tyre is a Passenger vehicle, with a 210mm width, an aspect ratio of 70%, Radial ply construction and a diameter of 12 inches.

Load Index - The load index indicates the weight in which your cars tyres are intended to carry, the number ois often provided between 0-279. Again, continuing hte example tyre number, P21070R12110 would indicate a weight carrying capacity of 110kg.

Speed Rating - Speed rating is indicated by a letter which corresponds to the maximum speed considered to be within its limits of safe operation. These letters range between A and Y, with A being the lowest and Y being the highest speed.

Additional Symbols - Some tyres may feature specialty characters to correspond to their specific functions, certain symbols and letters may include M+S which denotes that it is a mud and snow tyre.

Although at first glance all of these symbols and characters may seem like a big mumble jumble of nothing, being able to decipher the letters and numbers which are displayed on your tyres side wall will ultimately help you determine the tyre best suited to your desires and your cars needs.

Check out our video to learn more: