Car 101: what all those letters and numbers on the side of a tyre mean
Friday, 3 August 2018
Ever wondered what those mysterious numbers and letters on the side of your car's tyres are all about? Or, more importantly, what they should mean to you?
Well, wonder no more, because we will decode the mystery of your tyres.
The side of your tyre will have the brand name and model name in the biggest writing, but the arcane collection of letters and numbers (sometimes smaller, sometimes not) tell you more. They will look something like this:
These number represent, in order, the section width (225), aspect ratio (45), construction (R), rim diameter (19), load index (92) and speed rating (W).
The section width (225) is the width of the tyre in millimetres, in this case 225mm. This measurement is taken from the maximum width of the mid point of the tyre's sidewall to the same point on the opposite sidewall, so it's not just the width of the top of the tread, rather the maximum overall width of the tyre when looking down on it.
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The aspect ratio (45) or profile is the height of the tyre's sidewall expressed as a percentage of the tyre's section width, which in this case means 45 per cent of 225mm, or 101.25mm.
This is what is referred to when we talk about 'low profile' tyres on performance cars - the ones that look like a black rubber band stretched around a massive alloy wheel.
A low profile tyre generally has an aspect ratio of 50 per cent or below, down to as little as 30 per cent for ultra-low profile tyres on supercars and the like.
Lower profile tyres will provide greater control and better handling through corners thanks to less sidewall movement (the less sidewall there is, the less it can flex under load), but they will also give a harder ride due to that same lack of sidewall flex.
The construction (R) indicates the style of construction of the carcass of the tyre. R means 'Radial', which is by far the most common form of tyre construction today - literally 100 per cent of all new cars use radial construction tyres.
The others are mainly only for specialist or vintage cars and consist of B (denoting 'Bias belt' construction), D (meaning 'Diagonal') and if there is no code, then the tyre is a cross-ply.
The rim diameter (19) is the size of the wheel the tyre will be fitted to. It is measured in inches and is taken from the wheel flange where the tyre is seated to the same point on the opposite side.
The load index (92) indicates the maximum weight the tyre can take when travelling at its maximum speed rating, but not exceeding 210kmh. In this case '92' means 630kg per tyre, more than enough to hold a Mazda6 wagon up properly.
The speed rating (W) is represented by a letter and indicates the tyre's maximum speed capability when operating at its maximum load carrying capacity. In this case the 'W' speed rating, indicates it can be safely operated at a maximum speed of 270kmh at its maximum load, which will literally never happen with a Mazda6 wagon.
Basically, these number indicate the correct tyres for you wheel and car. Changing any of these ratios or sizes can have a massive effect on how your car handles and rides (both good or bad), so making sure they all line up is essential.