Getting your tyre size right when you are shopping for a new one is vital. Let's face it, you wouldn't go out shopping for a new outfit without first knowing the size of clothes you require -you'd probably end up getting the wrong size and the clothes would either not fit, or fall off. It's the same with tyres.
The best way to work out what tyre size you require if you are short on time is to go outside and at the tyres that are already mounted to your vehicle. Assuming that the car has been fitted with the correct size of tyres in the first place, all the information that you will require is printed on the sidewall of the tyre.
However, if do not trust that the tyres already on your vehicle are the correct size, you can simply look it up in the vehicle's handbook. This will be the correct tyre size.
What - for many people - makes buying new tyres confusing is that there are a large variety of car tyre sizes available.
A tyre size is generally shown in a mixture of numbers and letters. To give an example, quite a common tyre size is: 225/40 R18 W (88). This code breaks down as the following:
225 - The tyre width in millimetres.
40 - The height of the tyre sidewall as a percentage of the width (also known as the aspect ratio)
R - This indicates what the tyre's construction is. R stands for radial which all tyres must be in the present day.
18 - Indicates what the diameter of the inner rim is in inches.
W - This is the speed rating. The symbol indicates the max speed for the tyre at full load (more on speed rating below).
88 - This is the load index - it indicates the heaviest maximum load that the tyre can take.
From time to time you may see a tyre size code that has the speed rating symbol at the end of the line. For the above tyre example it would look like this: 225/40 R 18 88 W.
Speed Rating
Getting the correct speed rating for your tyres is crucial. It must match or exceed the total speed of the car. For example, if your car's top speed it 150mph, you will need to find a tyre with a speed rating that exceeds this.
A breakdown of the speed rating codes is as follows:
S - 180 km/h or 112 mph
T - 190 km/h or 118 mph
U - 200 km/h or 125 mph
H - 210 km/h or 130 mph
V - 240 km/h or 149 mph
W (ZR) - 270 km/h or 168 mph
Y (ZR) - 300 km/h or 186 mph
ZR - Above 240 km/h or 149 mph
So, using the example from above, this tyre would be suitable for a car with a maximum speed of 168mph or less.
