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Essential Car Tyre Guide

Everything you need to know about car tyres for safe driving

Four small patches of rubber each about the size of your hand are the only parts of the car in touch with the road. So, knowing as much as possible about tyres is essential for safe driving. Our car tyre guides cover everything you need to know.

The right tyres, in good condition and correctly inflated are vital for braking and cornering safety – and the only things keeping the car on the road.
Regular checks and maintenance will help to prolong the life of the tyres and keep you on the right side of the law too.

Tyre Life and Age is influenced by many factors:

Tyres are designed to meet different criteria.  Long life tyres are made from a harder compound and may be noisier as a result.  Others may be made from a softer compound which are quieter but will wear out quicker.  It’s often the case too that a car’s original tyres will last longer than later replacements.

Tyre life is influenced by several factors:

  • Driving style.  For example, aggressive cornering and harsh braking increases wear
  • Position.   Front tyres wear faster because of movement through steering
  • Speed.   High speed driving increases temperature and increases wear
  • Load. Heavy loading increases wear
  • Pressure.  Both under inflation and over inflation increase wear
  • Alignment.  Incorrect wheel alignment results in rapid and uneven wear.

Generally on a front-wheel-drive car you’d hope for a minimum of 20,000 miles for front tyres, and double that for those on the rear.

The legal minimum limit for tyre tread depth in Ireland is 1.6mm.  This is because, tyre performance, particularly in wet weather gets worse as the tread wears down.

The greater the wear to your tyres the worse its wet weather grip will be.  Tread depth should be checked more frequently once it reaches 3mm.  Tyres should be replaced before their tread depth wears below 2mm.  This is especially important if heading into autumn and winter months.

Tyre age

Age is a separate issue to wear.  Tyres deteriorate naturally through exposure to heat, sunlight (Ultraviolet/UV) light and rain. Degradation depends on the amount of exposure and the severity of the weather.

This is a more common problem for caravans, trailers and vehicles which are often only used occasionally.   In everyday vehicles, tyres almost certainly will wear out before they degrade to such an extent they’re unserviceable.

UV cracking tends to occur in tyres that are four or five years old.  Tyres should be replaced irrespective of age if cracking becomes severe

Tyre pressures are quoted for cold tyres and will be specified in your car manual. They may also be shown on a sticker inside the fuel filler cap or driver’s door.

Generally two figures will be quoted; one for ‘normal’ use and a higher figure for full loads.  You should make sure to adjust the pressures as required.

If you don’t know the correct tyre pressures you can contact the vehicle manufacturer’s customer services department.

Correct tyre pressure is important for several reasons:

  • Tyre life – under or over inflation increases wear
  • Fuel economy – under inflation increases fuel consumption
  • Safety – under or over inflation will affect grip and braking performance
  • Ride – over inflation can result in a harsh, uncomfortable ride

You should check tyre pressures every two weeks when the tyres are cold using a reliable tyre pressure gauge.

At the same time, examine the tyres for any cuts or bulges.  If you find any, have them checked out by a professional as soon as possible.  A bulge in a tyre indicates internal structural damage so the tyre will need to be replaced.

The tyre must also be replaced if you find cuts deep enough to reveal the internal structure of the tyre.

Punctures in the tread area of a car tyre can often be repaired if the driver hasn’t driven too far on it.

Tyre sealants

Some car makers supply a tyre sealant and inflator pack instead of a spare wheel. The sealant and compressed gas are injected through the tyre valve.  However the result will depend on the cause of the puncture and the distance travelled on the deflated tyre.

There are two basic types of sealant.

Pre-puncture sealants

These are put into the tyre as a preventative measure either at the time the tyres are fitted or by injection through the tyre valve.  The aim is to prevent air loss if a puncture does occur so the driver can continue the journey without interruption.

The seal is pretty much instantaneous after the tyre has been punctured.  The concern with this is that the driver will have no idea whether there’s been a puncture or not. A large screw or nail in the tread of the tyre will cause further damage over time and could lead to more serious problems.

Post-puncture sealants

These are used following a puncture. The sealant and compressed gas to re-inflate the tyre are applied through the valve. Carrying a can of tyre sealant can bring peace of mind if you regularly drive alone, but it is vitally important that you spot the puncture early and stop quickly.  Running in a partially or fully deflated condition will cause weakening of the tyre structure and irreparable damage.

Permanent or temporary?

Advice from tyre experts to users of pre-puncture sealants is to inspect the tyres VERY often, i.e. Every time you use the vehicle. If the pre-puncture sealant has been activated there should be signs of the sealant (usually white latex based) on the outside of the tyre. If there are any signs of this or of a penetrating object, then the tyre must be removed from the rim and properly inspected to see if it is suitable for permanent repair.

Run-flat Tyres have limited capability following a puncture.

Many car makers are now fitting run-flat tyres as original equipment.  These have either a reinforced sidewall or additional internal structure to provide partial support to the tyre in a deflated condition.

Ride quality on a deflated run-flat tyre will be similar to normal.  The driver may not be able to tell that a puncture has happened, particularly if driving on the motorway. For this reason run-flat tyres will always be fitted together with a tyre pressure monitoring system (TPMS).

Refer to the vehicle manual for restrictions on use following a puncture.  Deflated tyres should be capable of at least 80 kilometers at speeds up to 80 kmph and should be renewed or repaired as soon as possible after a puncture.

Worn or damaged, run-flat tyres should be replaced by the same make of run-flat tyre. In some cases the TPMS will need to be reset after the fitting of a new tyre

Filling Tyres with Nitrogen

The air we breath (and the normal compressed air used to inflate tyres) contains 78% Nitrogen, 21% Oxygen and 1% other gases.

Purified nitrogen has been used to inflate tyres on aircraft and racing cars for many years, but now some tyre specialists are offering nitrogen inflation for ordinary car and van tyres.

The advantages of using nitrogen in specialist applications are clear

  • Planes fly at heights where temperatures may be as low as -40C. Any moisture in the tyres can freeze causing vibration and balance problems when landing. Pure nitrogen is dry so eliminates this problem.
  • In motor sport the smallest fraction of a second can make the difference between winning and losing. Filling with nitrogen can reduce tyre pressure variation caused by changes in tyre temperature.
  • Reduced corrosion – because unlike air there’s no moisture in pure nitrogen
  • Slower rate of pressure loss – because nitrogen molecules are larger than oxygen molecules (which make up 21% of compressed air)

Leakage can occur through the tyre’s inner liner but can also occur through the valve, punctures, or failure of the seal between tyre and wheel rim. Pure nitrogen might leak more slowly through the liner, but regular checks of tyre condition and pressures will still be essential.

Corrosion of the tyre through use of normal compressed air alone is most unlikely because only the outer tread band of a car tyre contains steel.  The amount of moisture reaching it from the inside is minimal.

Changing to nitrogen involves removing all the air which is already in the tyres and then re-inflating them with purified compressed nitrogen. There will be a one-off charge per tyre but once filled with nitrogen any future top-ups would also have to be with nitrogen if any advantages are to be maintained.

Overall, while accepting the possibility of purified nitrogen being of benefit in certain applications, we don’t think that the cost and possible inconvenience are justified for normal passenger car use.

Tyres for Space Wagons/Multi Purpose Vehicles.

Heavy vehicles and loads place greater demands on the tyres.   They run hotter and would be more susceptible to failure if they were not of the correct specification.

Most Multi-Purpose-Vehicles (mpvs) are heavier and can carry more load than a car of similar size. As a result most MPV manufacturers now specify reinforced, extra load or commercial tyres.

Though it may not be mentioned specifically in the manual that the vehicle is fitted with reinforced tyres it’s important to make sure that any replacement tyres match the tyres fitted when the car was new.

Markings

To check if your vehicle is fitted with extra load capacity tyres look for the following markings on the tyre sidewall:

  • XL – Extra Load
  • RNF – Reinforced
  • C – Commercial

If you’re unsure which specification of tyre should be fitted to your car you should contact the vehicle manufacturer or one of their appointed dealers.

Original fit car tyres

For new cars the car maker and tyre manufacturers work closely to select a make, size and tread pattern that suits the car.  Styling, handling, noise and many other factors are taken into account.

When replacing your tyres you must stick to the same size, type and ideally brand and tread pattern too.  This is likely to be the tyre best suited to the car. Changing brand or pattern could result in increased noise or adversely affect cornering characteristics for example.

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New tyres to the front or rear?

Check the car manual first as some vehicle manufacturers give specific advice on this. If there is no information in the manual, then it’s good practice for safety to fit the best/newest tyres on the rear.  In wet conditions, this favours understeer rather than oversteer. So if you have the front tyres renewed it’s best to have the rear ones moved to the front and the new tyres fitted to the rear.

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Understanding car tyre ratings

Every new tyre sold in the EU since 2012 comes with a tyre rating. This gives you an insight into the tyre’s effect on fuel efficiency, stopping distance in wet conditions and the external noise pollution a tyre will have. A tyre is scored from A to G, A being the best and G the worst, while the external noise rating is measured in decibels (dB).

How tyres affect your fuel efficiency

With the rising costs of fuel, we are all looking to save on our motoring costs. Paying attention to a tyre’s fuel efficiency rating can do just that. This is assessed by the tyres rolling edge, showing how much energy you stand to lose from standard day to day wear. You want to aim for a tyre with a lower rolling resistance so closer to the A rating.

Another method to save on your fuel efficiency is becoming an AA member to start enjoying our fuel savings, you can save up to 3c a litre on petrol or diesel at selected circle k stations nationwide.

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