See those markings on the tyre sidewall? They are shorthand for a veritable wealth of information. The tyre model name is present and correct, obviously, but that’s just the start. There’s also a sequence of numbers detailing the load index, speed rating, tyre size, construction, and much more besides.
Why exactly are these markings so important? This information is necessary when the time comes 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.
Let’s start with the following sequence of tyre markings as an example: 225/45 R 18 95 H
The first number to appear in the sequence is “225.” This number is the nominal width of the tyre (in millimeters) from one sidewall to the other.
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.
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.
The number “18” in our example represents the diameter of the wheel rim in inches.
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. More information on the load index.
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. Read our guide to speed ratings.
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