**Markings the tyre sidewall offer a wealth of information, such as the load index, speed rating, tyre size and construction.**

This information is important when it comes time to fit your wheels with replacement tyres. Every driver wants to maintain optimal safety and performance when buying new tyres; understanding the sidewall markings is an effective way of achieving exactly that.

**Using the following sequence of tyre markings as an example: 225/45 R 18 95 H**

**Tyre width**

The first number to appear in the sequence is “225.” This number is the nominal width of the tyre (in millimetres) from one sidewall to the other.

**Aspect ratio**

Following the slash, the next number in the sequence is “45”. This number is the tyre’s aspect ratio – essentially the height of the tyre’s profile outward from the rim represented as a percentage of the tyre’s width. We calculate this number by dividing the tyre’s section height by the tyre’s section width. So, if a tyre has an aspect ratio of 45, that means the tyre’s height is 45% of its width.

**Construction**

Next in our series of tyre markings is a letter instead of a number. This letter indicates the type of construction used within the casing of the tyre, which in our example is “R” for Radial construction. Other examples are “B” for Bias-ply or “D” for Diagonal construction.

Radial tyres are the most common tyres on the road today. They’re called radial because the tyre’s internal ply cords are orientated in a radial direction, from one bead over to the other, at right angles to the direction of the tyre’s rotation.

**Rim diameter**

The number “18” in our example represents the diameter of the wheel rim in inches.

**Load index**

The number after the rim diameter represents the load index. In our example, “95” is a code for the maximum load a tyre can support when fully inflated. Passenger tyres have load indices spanning from 75 to 105, where each numeric value corresponds to a specific load capacity. The carrying capacity for each value is described in a load index chart in your vehicle or tyre documentation.

**Speed rating**

Last in our sequence, we come to the speed rating. Letters ranging from A to Z represent the speed rating. Each letter denotes the maximum speed a tyre can sustain under its recommended load capacity. In our example, “H” is equivalent to a maximum speed of 130 mph (209 kph). Even though a tyre is capable of performing at this speed, drivers should not exceed legal speed limits.

**Additional markings**

After this sequence of information, other letters and symbols may also be present on the sidewall.

In our example, they denote the following:

7 Self-Supporting Run-flat tyre

8 Mercedes Original Equipment key

9 Suitable for mud and snow conditions

10 Compliant with US Department of Transportation motor vehicle safety standards

11 Week of manufacture

12 Year of manufacture