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Tyre Glossary

We understand purchasing new tyres for your car can be confusing and that’s why we’re to help. As well as providing helpful information over the phone and via email, we’ve put together a tyre glossary to help our customers understand more about the world of tyres.



This relates to the aligning of the wheels. If your wheel alignment is out, it may be that when you are attempting to drive in a straight line, your car pulls to the left or right. Your tyres may also suffer from unnecessary friction and wear. The braking efficiency may also be effected. Correct alignment will prolong the life of your tyres and increase fuel economy. As well as tyre supply and fit, we also offer wheel alignment services from our tyre shop in Portsmouth.


Some tyres have different tread patterns on tyres that are designed to be fitted on either side of the car. This is done by tyre manufacturers to increase performance and handling. The inside part of the tyre tread pattern is usually designed to channel water away from the tyre and the outside part of the tread pattern is designed for optimal grip and handling. The highly-rated Goodyear Eagle F1 Asymmetric F2 tyres are an example of this tread pattern in use.


Balancing should always be carried out when you fit new tyres. Balancing ensures the weight of the tyre is distributed evenly when it spins on your vehicle. We carry out balancing by using a balancing machine that simulates the tyre fitted on the car and spins. We then add small weights on to the inside or outside edge of your wheel.


A unit of measure. Air pressure is sometime measured in Bar or PSI (Per Square Inch). For example 1 Bar equates to 14.5 PSI and 2 Bar is 29 PSI.


The bead is the round inside edge of the tyre. It consists of reinforced materials that help the tyre grip the wheel.

Exta Load (XL)

Some tyres are reinforced and approved to carry a higher load of weight over standard tyres. This gives the tyre the ability to be inflated to higher-than-normal pressures. Look for the XL marking for Extra Load tyres. Some models of cars come with recommendations from the manufacturer to fit XL tyres.

Load Index

The load index is the maximum weight a tyre can carry. This value can be found on the sidewall of the tyre and is usually in Kilograms.

Maximum Inflation Pressure

This is the maximum pressure a tyre can be inflated to. This marking can be found on the sidewall of the tyre. It would be dangerous to drive the vehicle, with more pressure than the recommended maximum inflation amount.

Original Equipment (OE)

Tyres that are selected by a manufacturer for particular models of their cars, would be classed as OE. Some manufacturers also use other markings to indicate a particular tyre is suitable for their vehicles, for example tyres recommended by Audi will feature AO in the tyre name, tyres recommended by BMW will feature BMW in the tyre name and tyres recommended by Mercedes will feature MO in the name. Other manufacturers may have other markings.


PSI means Per Square Inch. Tyres are usually inflated according to what PSI is recommended. Some car models recommend the same PSI in all 4 tyres, while other may recommend different PSI amounts for the two front or two back tyres.

Rim Size

Usually this is measured in inches. It is the size of the outside diameter of the wheel, for example on a tyre with a size of 225/40/18 - 18 would be the rim diameter in inches.


Some tyres come with a Run-Flat option. This allows the tyre to driven at reduced speeds in the incident of it having been punctured. Run-Flat tyres are only designed to be driven for a limited distance, but this would do one of two things. It would allow you to drive to a safe place to change the tyre and also protect the wheel from damage caused by the road and weight of the vehicle.


This is the section of the tyre from the bead of the tyre to the top edge of the tread. The sidewall is designed and in some cases, reinforced to help protect the tyre when it impacts with the curb. The sidewall also contains all the information about the tyre, such as the manufacturer, load values, rotational information, maximum inflation pressures, tyre profile, size, speed rating and width.

Speed Rating

The speed rating is usually a letter such as V or Z. The letter indicates the range of speed a tyre is approved to drive at. Some vehicle manufacturers will recommend a certain speed rating for their models.


The tread is the pattern you see across the surface of the tyre which makes contact with the road. Designs vary between tyre manufacturers and their range of tyre models. Many years of research are put into optimising tread patterns for use of the tyre in various climates and conditions.

Tread Depth

The tread depth is the amount of tread a tyre has remaining. Usually measured in mm (Millimetres). New tyres may come with a tread depth of around 6.0/6.5mm or more. The minimum tread depth required for all tyres by law is 1.6mm. (Correct at time of writing on 19/05/15) - Some tyres such as Michelin tyres come with visual tread depth indicators, taking out the need to manually measure your tyre tread depth.

How to Check Tyre Tread Depth

Tyre Pressure

The tyre pressure is simply the amount of air pressure inside a tyre. This is usually measured in PSI or Bar. Over-inflation of tyres can affect the braking efficiency and grip on the road, the same way under-inflation of tyres can reduce fuel economy and reduce braking efficiency and road grip. This is why you should stick to guidelines recommended by the manufacturer and find the right pressures suitable for your tyres and your vehicle.

How to Check Tyre Pressure


The valve is the small tube-like object that allows you to fill a tyre with air. You can also press the pin on the valve to allow air to escape from the tyre. At Portsmouth Tyres, we always fit new valves with all new tyres.

If you need tyres in the Hampshire area and would prefer to see or speak to someone for more advice, get in touch and we can help.

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Areas served around Portsmouth:

Portsmouth - Southsea, Milton, Fratton, Anchorage Park, Portsea, Buckland, Northend, Copnor, Stamshaw, Clanfield, Lovedean, Purbrook, Horndean, Gosport, Portchester, Bedhampton, Eastney, Gunwharf quay, Hilsea, Baffins, Cosham, Havant, Hayling Island, Drayton, Bedhampton, Purbrook, Waterlooville, Farlington, Fareham, Paulgrove, Petersfield and more..