Tyre Information, Codes & Sizes Explained

Tyres are an absolutely crucial part of safe motoring. What's more, tyre quality, tyre size, tyre speed ratings, and tyre pressure all have a huge impact on your vehicle's performance, your fuel economy, and your car's ability to deal with the Great British weather.

The good news is, all the information you need about tyres can be found printed or moulded on the tyre sidewall - and we're here to help you understand exactly what all that information means, including:

• Tyres sizes
• Tyre load index
• Tyre width
• Tyre speed rating

...and much more. So, if you're looking at replacing tyres and need to know which tyre size and speed rating to order, or you're just curious to know what all those different tyre markings mean - we've got you covered!

Why are tyres so important?

Most people get in their car without giving a second thought to their vehicle's tyres - but the statistics suggest that roads would be much safer places if we paid a little closer attention to the rubber that holds us to the road.

Consider these facts:

• Tyre defects are a contributing factor in almost twice as many accidents as mobile phones
• 3 points can be added to your licence of each illegal tyre on your car
• Tyre defects result in 2,200,000 MOT failures each year
• In recent years, 5,375 casualties and fatalities have come from tyre-related incidents

What's more, in heavy rain, a good tyre takes just 7 seconds to displace enough water to fill a bucket - but without adequate tread, a car at any speed could just glide across that water, leaving you dangerously out of control.

So, put simply - safe tyres are an important part of keeping you and others alive.

Tyre size

Even if 2 cars have the same size wheels, there's every chance they'll have different sizes of tyres.

Why?

Well, tyre size involves more than just the size across the wheel; you've also got to consider the width and the depth of the tyre.

As such, you'll usually see tyre size information presented like this:

245/50 R18

This tyre sidewall information gives you the width of the tyre in millimetres, the aspect ratio, the construction type, and the wheel diameter.

Let's look at each of those things in a bit more detail:

245 - The tyre width

The first three figures included in the tyre size information are is the width of the tyre from sidewall to sidewall - measured in millimetres.

50 - The tyre aspect ratio

The 4th and 5th number give the height of the tyre sidewall - also known as the 'aspect ratio'. It's referred to as an aspect ratio because actually a percentage of the tyre width - so in this case, the sidewall height is 50% of the width.

R - Tyre construction

Today, most tyres have a 'radial' construction - a term used to describe how the fibre cords inside the tyre are arranged when it's manufactured. In rare instances, you may also see 'B' (describing a 'bias belt' construction) - or 'D' (a diagonal construction).

18 - The wheel rim diameter

The final 2 numbers on a tyres size marking indicate the wheel rim diameter that the tyre is designed for. This measurement is in inches.

Additional tyre symbols explained

The information we've covered will usually be enough to order new tyres for your car - but there's plenty more info moulded into the side of your tyre - including a 'load index' code and tyre speed rating.

Let's take a look at what each of those things means:

Load index

After the tyre size info, you'll 2 or 3 numbers that represent the 'load index' of the tyre. The load index is an indication of the maximum load that this tyre can carry.

We've put a table together to show the different load index codes on most tyres in the UK - for example; if your tyre has a load index of 100 - the maximum load it can carry is 800kgs - or a load index of 108 means a maximum load of 1,000kgs.

Now, an 800kg load might not sound like much - but remember, the load index rating is 'per tyre' so the overall load that 4 tyres can carry will be significantly more. If you're not 100% certain about the load index required for your specific vehicle, it's a good idea to look in your handbook. Alternatively, your tyre retailer will be able to recommend suitable load-rated tyres based your car and your driving.

Speed ratings

After tyre sizes and the load index code, you'll find a single letter. This is the 'speed rating' of the tyre - and it means that this model of tyre has been tested up to a certain maximum speed.

Speed ratings actually start from A - but this lower speed rating tyres tend to only be used for rating the speed on specialist vehicles and machinery.

What tyre speed rating do I need?

Normal road-going cars usually require a tyre speed rating S or T - meaning the tyres are not safe to be used at a speed of more than 112mph or 118mph (180 - 190km/h).

For most law-abiding citizens, rating the speed of a tyre so high might sound excessive - but the speed rating on a tyre is more about what a vehicle is capable of doing - rather than the speed you'll actually be doing. As such, some high-speed performance cars will need V, W, or Y rated tyres - capable of 149mph, 168mph, or 186mph respectively. Should you be lucky enough to own a 200mph+ hypercar, you'll need tyres with specialist (Y) speed ratings - tested to a maximum speed of 186mph+.

Again, if you're not sure about tyre sizes or the maximum speed rating you require - your tyre retailer will be happy to help out.

What other tyre information is on the sidewall?

If you go and look for your tyre's size, maximum speed rating or rim diameter, you're likely to find a whole world of other information too. Some of this info's legally required - whereas some is just there to let you know who makes the tyre.

Take a look at some of the other information you might find on your tyre:

Brand and model name

Generally, the largest lettering on the tyre will be the manufacturer's name and the model name they've given to that particular tread pattern. Popular tyre manufacturers include Bridgestone, Pirelli, Michelin, Goodyear, and Continental - but there are thousands of different makes and models.

Country of manufacture

The country that your tyre was made in will also be indicated on the sidewall. Since tyres are large and heavy (and therefore expensive to ship), many tyre manufacturers have plants here in the UK - although it's not uncommon to find that tyres have been made in Europe or further afield.

Tread wear indicators

You won't find a tread wear indicator on the side of your tyres - instead, they tend to sit in the grooves of the tread itself.

You're looking for a small marker - sometimes the letters TWI - but it could be a small logo or pattern. Don't panic if you can't see one - they generally appear as your tyres get close to their 1.6mm legal tread limit. You should avoid relying on TWIs as indications that your tyres are low - it's always best to get to grips with measuring the tread yourself - or having a garage do it for you.

Manufacturing date code

Over time, rubber can perish and split - so it's useful to know that your tyres aren't too old.

Don't worry too much; checking tyre condition is part of your MOT - so you don't have to be an expert, but it's worth keeping the manufacturer date in mind if you're a classic car fan or purchasing an older pre-owned vehicle - particularly one with a low mileage, as this could mean it's still sitting on old tyres.

European ECE type approval code

An ECE code is proof that the tyre has passed European safety standards - as it's possible that some non-EU/UK-made tyres are not manufactured to the same safety standards. E11 means the tyre is approved in the UK. A Non-EU tyre may be missing other information too - such as load info or speed-ratings.

A little more on EU tyre ratings

As well as seeing EU safety approvals on a tyre sidewall, important information about your tyre's performance should be included on a sticker when your tyre is new - and many retailers/manufacturers show this information on their website too.

An EU tyre sticker looks like this:

Fuel economy

The section in the upper left part of the tyre sticker rates fuel economy - useful information when you consider that your tyres can account for up to 20% of your fuel consumption. Fuel economy is rated from A (green) to G (red) - although the D-rating is not used for passenger cars.

Wet grip

The upper right part of the sticker shows a wet grip rating - useful to know when the UK experiences around 150 days of rain each year! Wet grip is rated from A to F (although D and G are not used for passenger cars) - and it makes a massive difference to your ability to stop. On a passenger car applying full brakes at 50mph - A-rated wet grip tyres will stop you up to 18 metres shorter than a set of F-rated tyres.

Noise

Finally, at the bottom of the sticker there's a noise rating - showing the noise impact your tyres will have on your surroundings. A single black wave coming from the speaker icon indicates the tyre is 'quiet' (3dB or more below the EU limit) - 2 black waves is considered 'moderate' (between the EU limit and 3dB below) - and 3 black waves is 'noisy' (anything above the EU limit).

Why is tyre info so important?

Tyre information is really there to let you know if your tyres are fit for purpose.

You might not consider it, but there's a perfect tyre for you - based on your vehicle, the type of driving you do, the load you carry - and a host of different factors. For virtually all of us, the tyres your retailer picks will be perfect - but if you've got any specialist needs (a modified car, a car you use for track days, etc) then you might need to do a little research to make sure your choice is right.

Tyres are not something you want to cut and corners on. Choosing a lower speed rating than you need or a lower load tolerance could mean otherwise perfectly legal tyres become exceptionally dangerous.

While safety is the number one consideration when it comes to tyres, you need to consider your insurer too. There's a good chance that the small-print of your insurance documents will say you have to have manufacturer-recommended tyre sizes and speed rating on your vehicle - so if you make a claim and you're found to be running the wrong size or tyres with a lower speed rating or load tolerance, you could end up seriously out of pocket - or even prosecuted.

Getting your tyre pressure right

Even the correct tyres can be dangerous or damage your fuel economy if they're not inflated properly - so it's worth taking a moment to consider tyre pressures.

The pressure that your tyres need to be inflated to can differ significantly - not only because different vehicles weigh different amounts - but also because adding additional passengers and luggage can put different amounts of pressure on those tyres.

To find out your car's intended tyre pressures, you should check your handbook - or look just inside one of your car's front doors for a sticker like this:

Although some manufacturers vary - most apply this sticker when the car is constructed, and you can usually find it on the 'door jamb' - the section of bodywork underneath the B-pillar of the car. That's this section here:

The sticker might look a little complicated at first glance, but it's actually fairly simple when you know what you're looking for.

Understanding a tyre pressure sticker

Since the stickers tend to be the same for similar models, you'll need to pick out your tyre information from the list on the left. Then, when you have the right tyre size - you can read across to the right, where you've got 2 options: a 'light load' or 'fully-laden'. Since tyres give a little under increased pressure, you need them to be inflated a little more if you've got 4/5 people in the car and a boot full of luggage compared to having just 2/3 people onboard.

On the example above, you can see that there's around 3/4psi or 0.2-0.3bar difference between the pressures required for a light load vs those needed for a full load. It might not seem like much, but it can make a big difference to how your car performs and handles.

When it comes to checking your tyres, a tyre pressure gauge is helpful. You can pick one up for less than £5 - and they're really easy to use. In less than 2 minutes, you'll be able to quickly check the pressure on each tyre with your gauge. If they do need adjusting, you can do it fairly quickly with a footpump or a mini-compressor - or, if you've got some spare change; you can pay to use the airline on your local petrol station forecourt.

Although most modern cars have a tyre pressure warning system that should alert you if you have a puncture, they're not an exact science - and it's not uncommon for these pressure monitoring systems to be inaccurate. If you want to be 100% confident in your tyres, use a pressure gauge to check them weekly and before long journeys.

Buying part-worn or preowned tyres

There's no getting around it - replacing 4 tyres can be expensive - and you tend to find that the larger the tyres, the more costly replacements become. Today, it's not uncommon for sports or SUV vehicle to have 18-, 19-, or 20-inch wheels - and that can mean that 4 tyres could cost £600-£800+.

With a big bill in mind, it's tempting to look at second-hand tyres - or 'part-worn' as they're often advertised as.

Before you buy part-worn tyres, consider this:

Recently, not-for-profit safety organisation TyreSafe.org released findings of an investigation into part-worn tyres. They found that 97% were sold illegally - with 34% containing potentially dangerous forms of damage or non-compliance.

It's very difficult to tell whether a pre-owned tyre has been repaired, under-inflated, or even used on a written-off vehicle before it's sold as a part-worn - and if it has been repaired, there's very rarely any evidence to say it's been done to the necessary BS AU 159 standard.

Safe tyres save lives

Tyres are one of the most costly factors when you're running a car - but as TyreSafe says; safe tyres save lives.

Make sure you're running the correct tyre size, with the correct maximum speed rating - and always ensure the load index is correct for the kind of driving your doing.

Tyres are serious business - always seek professional guidance when you're buying - and don't be tempted to cut corners.