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Understanding your tyre

Find everything you need to understand every inch of your tyre, to ensure you're driving safely on the road.

Sidewall markings

Tyre sidewall markings help vehicle users to identify, purchase and correctly maintain their tyres. Initially they were only used to help identify the tyre's make. Over the years, Government legislation has demanded safety markings be posted in order to help buyers know they are getting a legal tyre.

There are now two main groups of tyre markings we need to be aware of:

  1. Those essential markings which tyre manufacturers have long used to indicate make, size and type (including recommended inflation pressure and direction of rotation)
  2. Markings which have been added over the years in order to respect the requirements of legislation in the principal areas where UK tyres are used: UK, USA, Australia and Europe
  • Width of tyre in millimetres

  • Profile height of a tyre sidewall as a percentage of the width

  • The diameter of the wheels inner rim in inches

  • Load Index indicates the maximum load the tyre can carry

  • Indicates the maximum speed for the tyre

  • The model name of the tyre. Unique to each manufacture

  • Maximum load that the tyre is designed to carry (in both kg and lbs)

  • Maximum pressure that the tyre is designed to be inflated to (in both bar and psi)

  • Brand name of the manufacture

Tyre jargon busted

We recognise that, for many people, entering a garage or workshop is like visiting another world. A world where the locals, although friendly and welcoming, do speak a bizarrely different kind of language. So, at all HiQ Centres, we always make a point of talking in plain and simple English. Because the last thing we want to do is make you feel uncomfortable or that you don't really know what's going on with your car.

However, vehicles these days are complex. So some understanding of the basics can save yourself a lot of confusion and concern - which is why we've put together our glossary of common terms and phrases. If you can't find what you're looking for here, feel free to pick our brains when you come in. Or get us to explain anything you don't understand if you speak to us on the phone. Our technicians are always happy to help.


This is a series of sensors that are used to prevent the car's wheels locking and causing a skid when you press the brake pedal. By automatically releasing and then re-applying the car's brake pads in rapid succession it allows you to maintain control while reducing speed in extreme circumstances.


These inflate immediately in an accident to create a cushion between you and any hard surface. Most new cars have an airbag in the steering wheel and a passenger airbag in the dash but other bags can be located in the seats or doors. A rear-facing child seat should never be placed in a passenger seat opposite an airbag because the force of the explosion which inflates the airbag could injure the child.


This refers to the correct vertical alignment of the tyre when your car is at its normal ride level. Having your wheels aligned ensures that they are all pointing in the right way. If the wheels are not aligned correctly, the car will not handle properly and it could adversely affect tyre wear and your fuel economy.


These are tyres that are designed to be used all year round, in any weather.


This is the name given to the effect you experience when your tyres cease to make direct contact with the road surface because of water (your car is, in effect, surfing). As a result, you loose grip and traction and your car can go out of control.


This is the ratio of the sidewall of your tyre to the width of the tyre tread and is designed to help you distinguish between tyres of different dimensions. Expressed as a percentage ie. A tyre of 220mm width with a sidewall height of 110mm would be a 50 Aspect Ratio, or 50 profile tyre.


This is the name of tyres with a tread pattern that must be fitted in an indicated orientation. Often a chevron shaped pattern which assists the removal of water under the tyre.


'Balance' is used to describe how a car feels. A balanced car is one that feels in harmony with its driver, whose front and rear wheels grip in an even manner. It also concerns the feel from the car's steering and the sensitivity of its controls, such as throttle and brakes.


This is the process of trueing wheel/tyre spin to remove the slight differences in a tyre and wheel assembly. This eliminates steering vibration and uneven tyre wear. To do this we use balancing weights that are discretely attached to wheel rims.


The area of the tyre that is in contact with the wheel rim. The bead carries a multi-layer steel band and a shape that helps hold the tyre on the rim.


This is an extra safety system in more advanced cars. It senses the speed of the driver's reaction and the attitude of the car and judges if extra braking is needed to compensate for you not having depressed the pedal fully.


This is the most common measure of an engine's power. It is calculated from the engine's torque (see Torque). The more able the engine is to push against a resisting force (essentially a brake), the more powerful it is, and hence the name brake horsepower. The name dates from the industrial revolution, when a steam engine was rated by how many horses it replaced.

BS AU 159F

The British Standard for tyre repairs for vehicles used on the road. HiQ carry out all tyre repairs to this British Standard.


Petrol and diesel cars all emit carbon dioxide (CO²) from their exhausts to varying degrees. The levels of these emissions are used to calculate company car tax.


Basically a more advanced version of air-conditioning. It monitors the temperature of the air inside the cabin and adjusts the air coming in to keep the cabin at a steady temperature.


A common (illegal) trick used by unscrupulous sellers to obtain a higher price. Research shows that 1 in 12 vehicles have some sort of discrepancy. The average car does 10,000 to 12,000 miles per year, so if you think the mileage is inconsistent with the age and condition of the car it is worth investigating further. Ask to see its service records, old MOT certificates and all other documentation.


This is the practice of changing a vehicle's identity - usually to disguise a stolen car. Crooks often use a registration plate taken from a different vehicle. Sometimes only the registration plate is changed, but often the VIN (Vehicle Identity Number) or chassis number will also be swapped with that of the 'donor vehicle'. Affected vehicles can be hard to detect, which is why it's vital to check that ALL a vehicle's vins - stamped into the bodywork, on the chassis plate and behind the windscreen match each other.


This is the average fuel consumption figure for your car. To get this figure, a car's official fuel consumption during town driving and out of town driving (e.g. Motorway) is added together and divided by two to give an average figure. This is the most frequently quoted economy figure because it best reflects the sort of driving most people do.


Cut 'n shut is the term used by the motor trade for a car that is made up of two cars, usually both 'write-offs'. The back end of one is welded to the front end of another, often with considerable skill. The car may look like new, but it is likely to be unroadworthy, could be lethal in a crash, and may be worthless when you come to sell it.

Things to watch out for are weld marks in the floor of the car and uneven gaps between panels and around the doors.

Also carefully check the car's documentation and make sure the chassis and engine numbers all match those on the registration document and MOT certificate.


An engine contains a number of cylinders inside which the pistons move up and down. As the piston moves down, it creates a chamber into which the fuel and air are drawn in and as it moves up again, these gases are compressed. A spark from the spark plug then ignites the gases and a contained explosion forces the piston down again creating driving force. The burnt gases are expelled from the cylinder as the piston rises, and the cycle is repeated. The number of cylinders an engine contains dictates how much combustive power it can generate.


These prevent a door from being opened even if someone breaks the glass and reaches inside. Once activated, deadlocks can only be re-opened with the car's remote control or key.


This is the process where a tyre that is faulty, or has been run under-inflated, may break down, causing its tread layer to become separated from the casing.


This is the process by which dealers or garages plug their computer into the car's engine-management system to trace and repair faults.


A more efficient way of burning fuel by injecting it directly into the combustion chambers of an engine rather than a separate pre-combustion chamber. It is most common in diesel engines, although direct injection petrol engines are becoming more widely available.


Tyre tread patterns which vary from one side of the tread to the other, ie, they are not symmetrical. If these tyres are non-directional they must be fitted with the outside sidewall on the outer face of the wheel. If they are also directional, they will also be specific to the left and right sides of the vehicle.


The turning shaft which delivers engine power from the gearbox to the wheels of a car. It can be expensive to put right, so check it on any used car test-drive. In front-wheel drive cars, put the steering on full lock and pull away. If you feel a 'clunk' then the shaft could be worn.


The Government body that issues driving licences and registers vehicles for use on UK roads. Records of both are kept so that drivers and vehicles can be traced, for law-enforcement purposes or safety-related recalls. The DVLA collects road tax and also sells number plates.


All car tyres sold from 1st July 1997 must carry an 'E' marking. They certify that the tyre complies with the dimensional, performance and marking requirements.


The various gases that are pumped out of a car's exhaust pipe include carbon monoxide, nitrogen oxides and hydrocarbons, although the one to worry about for company car users is carbon dioxide (CO2). The more CO2 your company car produces, the more tax you pay for using it.


The total volume of all the cylinders in a car's engine added together and expressed in cubic centimetres (cc) or, more frequently, litres. So, an engine with a capacity of 1598cc will be referred to as a 1.6 litre unit. Generally, the bigger an engine's capacity, the more performance it will be able to produce.


An electronic box which monitors the operation and performance of your engine. Sophisticated braking and traction aids may also be linked to the management system so that it can coordinate their operation with that of the engine.


The area of tyre tread in contact with the road at any given time. This varies during acceleration, braking and cornering. It can be affected by tyre pressure, temperature, rubber compound, and tread design.


Cars which use the engine to drive all four wheels are described as having a four-wheel-drive transmission. This gives better traction in slippery conditions and is essential for off-roaders. However, it adds to the car's weight and increases fuel consumption compared to an equivalent two-wheel-drive car.


The most commonly used way of delivering fuel into the engine's cylinders these days. In older cars, fuel and air are mixed in carburettors before being burnt in the engine, but in new cars, the mixture is squirted in (injected) under high pressure. This helps make modern engines more efficient and produce lower emissions.


Before buying a used car, ask to see the car's service book. If this shows the car has been serviced on time and according to the manufacturer's instructions, your potential buy has a full service history. It's a sign the previous owners have cared for the car properly, so it is more likely to be reliable. This also adds to its resale value.


Most cars have the choice of a manual or an automatic gearbox. The former gives you more control, while an automatic gearbox, which changes gear for you, will be popular with those who want an easier drive.


Short for Global Positioning System. Or, in other words, satellite navigation (SATNAV).


Lateral - drains water to sides of the tyre. Circumferential - evacuates water to the rear of the tyre or "stores" it. The number of grooves increases as the tyre width increases.


This is how a car behaves when travelling through a corner. It is to do with how much traction the car has, how much it leans and how the steering feels in your hands. Good handling is about more than just keen driving, it's about keeping you on track when you discover a patch of wet leaves on a roundabout on a rainy night.

Ideally a car's handling should be neutral. However, most tend to understeer or oversteer depending on their weight distribution and which wheels are driven.


A background check should reveal whether a car has been stolen, is an insurance write-off, or has any outstanding finance secured against it. Having a history check is highly recommended when buying a car.


A hybrid car is one that uses more than one source of power. Unlike a bi-fuel car, however, which requires the driver to swap between power sources, a hybrid can swap between them as necessary while on the move, and can even make use of both when full power is required.


A major security device to prevent thieves from stealing a car, even if they gain entry. An electronic immobiliser is part of the engine control unit (ECU), and prevents the car from being started unless it recognises signals from a transponder in the key. Every new car in the UK has to have an electronic immobiliser fitted by law. You can also protect your car with a mechanical immobiliser such as a steering lock.


A type of suspension that permits your car's wheels to move independently of one another. That means when one wheel hits a bump in the road, the others are not affected. Very few cars do without it these days; the exceptions are usually serious off-roaders.


This is used with a turbocharger, which boosts an engine's power and efficiency by forcing compressed air into the cylinders. When the air is compressed by the turbocharger, the molecules are forced tighter together, which causes friction and heat. An intercooler increases power further by cooling this air.


This is shown on the sidewall of the tyre to indicate the maximum load the tyre is designed to bear.


The V5C registration form which identifies details of the car and the person recorded as its registered keeper by the DVLA in Swansea. When you sell a car, you and the buyer are both required to inform Swansea of the change of ownership.


Tyres with shallow sidewalls, fitted to high-performance or sporty cars. Typically, an average family car will have 65-profile tyres, which means the sidewall height is 65% of the tread width. Performance cars can be found running on 50-, 45- and even 35-profile tyres.


A secondary gearbox that takes drive from the normal gearbox and lowers the gear ratios so the road wheels turn more slowly than in normal use. This allows for greater traction when climbing very steep terrain or driving off-road.


This is an alternative to petrol and diesel, but is usually used in conjunction with petrol in bi-fuel cars. The LPG is housed in a separate tank in the boot and the driver is able to switch between the two sets of injectors (one for petrol, one for gas) on the move using a dash-mounted switch. LPG gives worse economy than petrol or diesel, but pollutes less and costs much less, so it is a cheap option.


The MOT test is a yearly inspection of every car that's three years old or more. The system is run by the Driver and Vehicle Standards Agency (DVSA). Its purpose is to ensure that vehicles meet legal standards of mechanical safety and also that they comply with emissions regulations. The document is also needed when you renew your road tax.


The original brand of tyre or battery that was fitted by the car manufacturer. (Original Equipment Manufacturer.) There may be more than one OE brand fitment.


This is loss of rear-end traction. If a car oversteers, the rear wheels slide wide, causing the car to corner more tightly than intended (i.e. It over-steers), and there is a risk it may spin. Typically these cars are rear-wheel drive, and enthusiastic drivers enjoy 'driving sideways'. However, it takes skill to control this trait.


When rearward vision is poor, parking distance sensors come into their own. Usually mounted in the bumper, these sensors detect any object close to the car and a beeping noise warns the driver, becoming more rapid as the car moves nearer.


Diesel engines emit tiny pieces of carbon, known as particulates, which are harmful to the lungs. Particulate traps (or filters) virtually eliminate these emissions, although diesel fuel with a very low sulphur content is needed for the system to work at its best. Peugeot and Citroën have led the way in the introduction of this technology.


The basic underpinnings of a car - the 'platform' - is often shared between various models and even between different car manufacturers to keep down costs. Cars which share the same platform do not necessarily feel the same to drive, however. Different engines and careful fine tuning of suspension settings can make similar cars feel very different once you're behind the wheel.


The ratio of the sidewall to the width of the tyre tread expressed as a percentage. So a tyre of 220mm width with a sidewall height of 110mm would be a 50 Aspect Ratio, or 50 profile tyre.


The process of cutting new grooves into worn tread to extend the life of truck tyres. This is an illegal operation on car, 4x4 and light commercial tyres.


A recycling process where a tyre is buffed back to its casing, repaired and rebuilt using fresh tread compound.


The process of replacing the tread layer on a tyre. Generally reserved for commercial, agricultural and industrial tyres.


A tyre designed to run at low or no pressure to enable the driver to reach a place of safety, or even complete the journey.


To ensure safety and reliability, cars should be serviced as directed in the manufacturer's handbook. A dealer stamp should be entered into the car's service book to show the necessary checks and work have been done. Failure to service a car and maintain its service history can have a negative impact on its used value.


The side of the tyre between the rim and the tread. This carries all the tyre identification data, load rate, speed rating and manufacturing data.


Narrow slits in the tread blocks designed to pump water away and to create more leading edges to increase grip in winter and wet-weather tyres.


A spare tyre of a smaller size than the road tyres. The purpose of the reduced size is to save space and reduce weight. Designed to get you home, these tyres have a slower maximum speed and are designed to be only used on a temporary basis.


Fuel with a higher octane content than regular unleaded and usually recommended in tuned performance engines. Failing to use super unleaded where recommended will not damage an engine but it will mean less power output. This fuel usually costs around five pence per litre more than regular unleaded.


Fuel with a higher octane content than regular unleaded and usually recommended in tuned performance engines. Failing to use super unleaded where recommended will not damage an engine but it will mean less power output. This fuel usually costs around five pence per litre more than regular unleaded.


Arrangement of springs, dampers and anti-roll bars which dictate how a car rides and handles. A successful set-up strikes a happy balance between comfort and cornering ability.


Sport Utility Vehicle, the US term for a 4x4 vehicle with boxy, off-roader styling, whether or not it can tackle serious mud.


This is the force from the engine at a given rpm, and the ability to move weight a certain distance, therefore measured in lb ft. Also known as pulling power. Torque is often more relevant when assessing a car than outright power, because you need to know how well a car can pull away from standing, climb a hill or accelerate past a slower vehicle.


Your tyres have grooves moulded into them to help them grip the road, and the tread is the area which actually touches the road. Tread depth is the depth of the grooves. Legally, the tread must have a depth of at least 1.6mm across the central three-quarters of the breadth of the tyre and around the entire circumference. If the tread is allowed to wear down further than this, the tyres will not be able to grip effectively, and you could suffer a skid leading to an accident, especially in the wet.


The process where a tyre that is faulty, or has been run under-inflated, may break down, causing its tread layer to become separated from the casing.


This indicates the level of equipment which comes with the car. The trim level is usually indicated by several letters such as LX or GLX. The higher the trim level, the more equipment will be included in the price.


The distance a car travels to complete a full circle with the steering wheel at full lock. Two different measurements can be quoted, either between kerbs, which looks at the distance travelled by the wheels, or between walls, which considers the width of the whole car.


Tyre pressure is the measurement of the volume of air inside the tyre when it is inflated. Formally measured in pounds per square inch (psi) now measured in bar. (1 bar = 14.503774 psi). Pressure will gradually reduce and therefore regular checking and topping-up is necessary to maintain the vehicle manufacturers recommended pressure. (See vehicle owners manual). Never inflate a tyre to a greater pressure than the maximum that is indicated on the sidewall of the tyre.


Understeer is loss of front-end traction, so a car will tend to veer wide of the intended line and ploughs straight on (i.e. It under-steers). This is a characteristic of modern front-wheel-drive cars. To correct it, drivers need to gently lift off - a natural reaction, and so most cars are designed to understeer rather than oversteer for safety.


Valves let fuel and air in and out of the engine's cylinders. In most cars, the timing of the valves' opening is fixed to offer a fair compromise across the whole rev range. VVT, however, adjusts the timing with the speed of the engine so that it is always working at its peak efficiency.


Vehicle Identification Number. This is the manufacturer's own ID, found under the bonnet, under the carpet by the driver's seat and/or etched on to the windows. You will also find it stamped onto a plate, either under the bonnet or in one of the door openings. Some cars have 'visible vins' behind their windscreens. Vehicles which are approved for sale in the EU have 17 digit vins.


Vehicle Registration Mark, or number plate.


The distance between a car's front and rear axles, usually measured in millimetres.

Keeping you on the road with our tyre tools

Find out what the recommended tyre pressure is for your vehicle, or find your next set of perfect tyres by using our handy tools below.