A Bit on the Sidewall – What Car Tyre Markings Mean

All car tyres have them but not everyone knows what the markings moulded in a tyre's sidewall means. At the very least, it is necessary on many cars to check the sidewall as the tyre pressure to be used can relate to the tyre size. However, this little trip through the meaning of the markings will inform, especially when the time to buy new tyres arrives.

Here, we can use a theoretical, typical car tyre. Let's say it's marked 185/55R13 91 H. Confused already? Don't be, things will become perfectly clear! Take the tyre markings one by one….

185 – This is the width of the tyre’s section in millimetres, i.e. the width of the carcass of the tyre, not its tread's width. 185 millimetres is a little over seven and a quarter inches.

55 – This figure refers to the tyre’s aspect ratio, which is the height of the tyre height expressed as a percentage of its width. 55 is a low-ish profile but aspect ratios can go as low as 30%. Low profile car tyres grip well and deform little but the cost is reduced ride comfort and poor protection against potholes, kerbs and road debris.
R – This couldn't be simpler. Here, R means radial.
13 – This number refers to the diameter of wheel the tyre will fit. Why this is expressed in inches rather than millimetres isn't clear but it is standard practice.
91 – This is the tyre load index, a code for the maximum load capacity of the tyre when driven at its maximum rated speed. The figure 91 means a rating of 615 kilograms.
H – This final marking shows the tyre’s speed rating. This H rating means the tyre can cope with extended use at a maximum speed of 130 miles per hour (210 kilometres per hour). The highest possible speed rating is Z. Car tyres thus marked are capable of maintaining 150 mph (240 kph).
There are numerous other markings on a car tyre's sidewall, some of which are for the tyre fitter's information. For example, if the tyre has a specific direction of rotation, there will be an arrow. A 'TWI' marking denotes where the tread wear indicator(s) are moulded into the tyre's tread. A coloured dot will show the tyre fitter where the tyre casing should sit on the wheel in relation to the valve. There will also be ECE (not EEC) type approval markings, possibly special construction markings, such as M&S (Mud and Snow), and a maximum pressure marking.
One marking that could be of interest to you is the DOT code. The last four numbers in this show the tyre’s date of manufacture. Four-digit codes began after 2003 so if your new car tyre has the code 2 5 0 9, it was made in the 30th week of 2009. Tyres over 6 years old must never be used.
All very interesting but do these markings matter to you? Apart from the aforementioned tyre size vs. tyre pressure element, the speed and load ratings are extremely important. Why? Because at best, using under-specified tyres is illegal and can invalidate your car insurance. At worst, the wrong kind of tyre can fail and cause a potentially lethal accident at high speed.

Article Resource

Merityre.co.uk are one of the leading UK independent suppliers of car tyres. Why not visit their website for an online tyre quote or contact your nearest fitting centre.