Deciphering your tyres

If you can read your tyre’s sidewalls then you’ve got access to a world that most people never notice.

Here’s our guide to understanding what your tyres are telling you.

Radial or crossply

While you can still buy crossply tyres for use on vintage cars, all modern cars use radial tyres. So, when you see ‘radial’ or ‘R’ marked on the sidewall of your tyres, you now know what it means.

Size

Every tyre has its size marked on the sidewall. Interestingly, the sizes are generally shown in a mixture of metric and imperial measurements, but then as Brits we’re used to that, aren’t we? (Do you, like me, think of hot weather in terms of °F but cold weather in °C?)

A tyre with 165x16 on the sidewall is 165mm wide across the tread, and fits a wheel that is 16 inches in diameter. The width figure almost invariably goes up in increments of ten (so 165, 175, 185 etc.) while the wheel diameter size goes up one inch at a time.

Most tyres have another pair of digits to the measurements though. So, if your tyres read something like 215/70x17, then this means that the tread width is 215mm and the tyres will fit a wheel that is 17 inches in diameter. The ‘70’ (the sidewall height figure can be an increment of ten or five, so can be 70 or 75, for example) simply means that the height of the tyre wall is 70% of the width, in this case 215mm.

This means that the height of the tyre sidewall in our example is 150.5mm. No-one ever really needs to calculate this but it’s useful to know the profile percentage for two reasons.

The first is that the lower the number for a given width, the lower the height of the sidewall. A lower sidewall can help improve a car’s handling because the tyre sidewall has less flex in it, which makes the steering feel a bit sharper. However, there is no such thing as a free lunch because a lower sidewall also makes the ride harsher than the same width tyre with a taller sidewall.

The second reason is because the height of the sidewall is one of the factors that determines the tyre’s rolling diameter. This used to be known as the Pirelli Plus One concept, because those of us who drove fast hatchbacks in the eighties used it as an easy way to work out what size tyres we’d need to choose to keep the car’s overall gearing the same.

So, if we take my old Golf GTI for example, the factory tyres were 175/70x13. I replaced them with 185/60x14 tyres (on 14-inch rims, obviously) and the astute among you will have noticed that while the width and wheel diameter went up in one increment, the sidewall profile percentage had to go down one to keep the overall rolling circumference the same.

I know what you’re thinking, but please do bear with me because this has a real world application even if you have no interest in tuning or modifying your car.

Let’s assume you are looking at buying a car; it may well be offered with a choice of different wheel sizes, generally in the 16, 17, and 18 inch range. Car manufacturers will rarely go to the effort of changing the gearing on the same model, so they need to keep the rolling circumference of the tyres they’re offering the same across the board.

This means that a 16-inch wheel and tyre combination will have taller sidewalls than a car that sits on 18-inch wheels; the former will be more comfortable and the latter will probably handle a bit better. This helps you factor in another dimension to your decision making in addition to simply deciding which to go for on the basis of which you prefer the look of!

If it helps, most keen drivers prefer the smaller wheels with the taller sidewalls because even though the ultimate grip they offer might be a bit lower than the wider tyres, those tall sidewalls with their added flex are more progressive when they do let go; they might grip less but they have more feel, which a lot of us prefer.

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Load index

The load index is expressed as a two-digit number. It is usually placed after the number that denotes the diameter of the wheel and before the letter that indicates its speed rating.

Black Circles has a handy table here but really all you need to know is the rating your car manufacturer lists as being suitable for your vehicle and then make sure that you don’t fit a tyre with a lower capacity than that.

Speed rating

Every tyre has a speed rating too, and it is usually shown immediately after the letter that indicates the tyre’s load index.

So, if your car has a top speed of 140mph, you need a tyre that is rated to at least that speed. In this case, you’d be looking at a ‘V’-rated tyre, which will cover you up to 240km/h, or 149mph. Again, Black Circles has a handy table here.

You can fit tyres with a lower speed rating than your car is capable of if the tyres are special all-season or winter tyres with an M&S marking on them. If you do decide to fit tyres with a lower speed rating then you must also fit a sticker on the dashboard of the car that can be easily seen by the driver reminding them of the rated speed of the tyres.

The car must not be driven above that speed and you should probably inform your insurance company, too.

Snow symbol

Tyres that are designed to be used in snowy and winter conditions will have the so-called ‘Alpine’ symbol. This is a snowflake inside a mountain with three peaks. You will sometimes hear this referred to as a ‘3PMSF’, which is shorthand for three-peak mountain with snowflake.

This is a legal definition and means that the tyre will perform well in snowy, icy and cold conditions. Some European countries insist on proper winter tyres in some conditions and seasons, and you may well be fined, or turned back if you’re trying to get up to a ski resort, if your car doesn’t have tyres with the 3PMSF mark on them.

M&S tyres

The old way of denoting winter tyres was to use a M&S (or M+S, or M.S.) marking on the sidewall. This mud and snow marking had no legal definition and was sometimes used more as a marketing tool than a reliable indicator of a tyre’s performance in winter or muddy conditions.

The M&S marking can still be used but you need to be aware that it doesn’t really tell you much about a tyre’s performance in adverse conditions unless it is accompanied by the 3PMSF symbol.

Build date

Every new tyre must have a four-digit code to show the month and year of manufacturer. The digits are usually shown within an oval and will be in the format ‘1217’ or ‘XXX1217’ (where ‘X’ is a letter). The numbers 1217, for example, indicate that the tyre was made in the twelfth week of 2017.

This is a useful trick to know because old tyres are dangerous and can’t be relied upon. So, if you’re buying a secondhand car, check the date they were manufactured and if they’re more than three years old, use that fact as a bargaining chip to get money knocked off because you’ll need to buy a new set.

Similarly, some disreputable tyre outlets will offer some of their wares very cheaply because the tyres are old and beyond their serviceable life, even though they might never have been fitted to a vehicle.

If the tyres on your car only have three digits and a triangle, then that means they were made in the 1990s and so should be replaced immediately on the basis of age even if the tread looks fine.

Tread wear indicator

All tyres have tread wear indicators (TWIs) built into the tread that show when your tyres are worn and should be replaced. However, did you know that there is a TWI indicator on the sidewall that shows the location of the TWI on the tread of the tyre?

(The TWI itself will be set to 1.6mm but I’d recommend changing your tyres when they’re down to 3mm.)

Oh, and Michelin tyres don’t have the letters ‘TWI’; they have the Michelin man instead, which is a charming little Easter egg to find!

Tyre pressure information

The maximum tyre pressure that the tyres will withstand is marked on the sidewall too. This is the maximum though, and so no use if you’re trying to establish what pressure to set your tyres to.

Your car’s handbook will tell you what pressure to use and there is usually a sticker inside the driver’s door shut if you can’t find the owner’s handbook.

However, if all else fails then 30psi (or 2 bar if you prefer metric) is a good starting point until you can contact a dealer to find out the precise figure should be.

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Balancing information

I bought a new set of tyres recently and noticed that all four had both a yellow and a red dot on the sidewall. I was extolling the virtues of the chap who had fitted them until my friend told me that he obviously didn’t know his stuff because the tyres hadn’t been mounted properly.

He went on to explain that while conventions vary from one manufacturer to another, the yellow dot (or white or blue, depending on whose tyres they are) will generally indicate the lightest static balance point of the tyre. This means that the yellow dot should be fitted next to the tyre valve, which is the heaviest point of the wheel and so needs as much help as possible if you are to avoid the need for heavy wheel weights to balance the wheel and tyre combination.

The red dot, by way of contrast, shows the point at which the tyre is most imbalanced. It might also show the highest point of the tyre if it is not perfectly circular. This is interesting but not desperately useful unless your steel or alloy wheels have a dimple on them to show their lowest point. If yours have such a mark, then you should have your tyres fitted with the red dot next to the dimple.

Treadwear, traction and temperature markings

Your tyres should also have treadwear, traction and temperature markings on their sidewall. This should read something like ‘Treadwear 200 Traction AA Temperature A’.

In this case, the treadwear index is 200. The index is easy to understand because a tyre with a rating of 200 wears half as fast (or should last twice as long, if you prefer to think of it in that way) as one that is marked 100. A high number isn’t automatically good though, because a tyre with a high marking might wear well but grip less well than a tyre that has a lower number. As we’ve noted before, tyres are like life: you don’t get anything for nothing.

The traction marks, which show how well your tyre grips the road, run from AA, A, B and then C, with AA providing the greatest traction and C the least.

Temperature markings are A, B or C. A resists and disperses temperature and heat build-up the best, while C is the worst.

Uni-directional tyres

Uni-directional tyres must be fitted the right way round and so have a direction-of-travel indicator on the sidewalls. You might think that this is blindingly obvious, but you’d be surprised at how often the fast-fit places get it wrong…

Tubeless tyres

Your tyres will also probably be marked as being ‘tubeless’. This simply means that they shouldn’t be fitted with an inner tube. Interestingly, some bicycles are fitted with tubeless tyres too.

Miscellaneous stuff

Your tyres might have other stuff marked on them too, all of which is of only minor interest. However, in the interests of completeness, I’ll jot down the two most important ones!

A ‘P’, or no letter at all, indicates that your tyres were designed for use on passenger vehicles rather than commercial vehicles and all new tyres offered for sale in the EU will have their EU approval mark and number on them.

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