Motoring Advice

What should your car tyre pressure be?

Sep 5, 2018

What should your car tyre pressure be?

You know that your tyres are the most important part of your car, given they’re the only physical contact point between it and the road surface, and you know that you have to keep them legal when it comes to the depth of the tread (the cut/shaped blocks of rubber that form the rolling surface of a tyre), but surely it doesn’t matter if they’re not quite inflated to the correct pressures set by the car manufacturers, right?

Wrong. Tyre pressure is almost as important to your safety (and your wallet) as tyre tread depth, so read on to find out how to keep your tyres in tip-top condition by keeping them properly pumped up.

Why is tyre pressure important?

As much as how much tread is left on your tyres, the pressure they are operating at is vital to their overall effectiveness. If you have them over-inflated, then the tyres ‘balloon’ in shape and wear unevenly down the centre of their contact patch, while the car will have less of its four rubber corners pressed down onto the road surface – meaning it will feel more nervous, bouncing around the road and proving difficult to control in corners. The car will also have less traction and increased stopping distances as a result, making it less safe to yourself and other road users.

Similarly, under-inflated tyres are just as bad. This time, the outer edges of the tyres’ contact patches will wear first and the lower air pressure in each tyre is like having a ‘flat’ – the car will therefore be using more fuel just to make progress, as there is a drag effect on each under-inflated tyre, so you’ll be spending money unnecessarily on more regular fuel stops. Under-inflated tyres also have the same safety impacts as over-inflated tyres, such as longer stopping distances, reduced traction and also more chance of aquaplaning in wet conditions – this is because the central tread blocks are not properly in contact with the road, so your vehicle will not disperse standing water but will instead skim over the top of it; this is known as aquaplaning and it’s very, very dangerous and often results in an accident of some sort.

How do I check tyre pressure?

Well, if the tyre is severely under-inflated (i.e., it has gone flat or has a puncture), it should be blatantly obvious; there will be very little sidewall (the part of the tyre you can see when you stand to the side of the car, looking at the face of the wheels) on show underneath the alloy wheel, and what tyre is there will be flattened out ‘pancake-style’ on the road surface, making it wider than the rest of the wheel. If its total deflation is as a result of a puncture, it will either need repairing or, preferably, replacing altogether with a new tyre.

Over-inflated tyres are harder to spot, but look to see if the tread in the middle of your tyre is worn more than the outer edges as a key sign, while also check for blisters on the sidewalls – these look like small, egg-shaped bulges in the tyre’s structure and, if these are displayed, then the tyre is already beyond saving and will need replacing.

If you think your tyre pressure might not be correct and you’re not sure how to verify it, simply head to a big fuel station or a tyre-fitting company. Either of these will have tyre inflation machines. These are simply compressors that can pump air back into your tyres, but they can also check the current pressures of your tyres, displaying them on a small digital screen. At a tyre-fitting company, you’ll normally have to pay a small fee to have someone inspect your tyres for you and use the compressor machine if required, so it’s best (cheaper) to seek out a compressor at a fuel station and do it yourself.

How do I know what the correct tyre pressure for my car is?

Most vehicles will have their tyre pressures listed in a printed panel that will be stuck somewhere on the car – the most obvious place for these is in the door jambs; when you open the front doors, you should see these tyre pressure charts on the area of the car’s body that has been exposed now the door is open. We have heard of some tyre pressure charts being displayed underneath the fuel filler cap, so if you’re struggling to find your chart in the doors, look there instead. Failing that, the tyre pressures of your car will all be printed in the owner’s manual and will be clearly signalled in the manual’s content pages under ‘Tyres’.

These charts, wherever you find them, will show the pressures for all sizes of wheel and tyre available for your model of car, and they will likely be shown in either pounds per square inch (known as ‘psi’) or ‘bar’, or – more commonly – both. Typically, passenger cars have tyres that run at 30-33psi or 2.0-2.2bar, but please do check your own tyres to ensure you’re getting the right pressure.

To know which pressure you need, you need to check some information. First of all, are you driving in the car alone or with a single passenger, while the boot is empty, or are you driving in a fully laden car with a load of luggage in the trunk? You need more air in your tyres for the latter scenario and the relevant pressure charts in the door jambs/fuel filler caps/handbook will show you the pressures accordingly, with simple infographics of one/two people or four/five people plus cases to identify the right pressures for your car’s load.

Then you need to know your tyre sizes. These are always printed on the sidewalls of the tyres themselves. They are displayed in the same format on all cars: the three-digit width of the tyre in millimetres, then a slash (‘/’), then the two-digit aspect ratio (this is the what the sidewall’s depth in millimetres is as a percentage of the tyre width – e.g., 225/45 means a 225mm wide tyres with a 101.25mm-deep sidewall, it being 45 per cent of 225mm), then the letter ‘R’ (for Radial construction, as cross ply tyres disappeared many decades ago) and then another two-digit number – this last one is the diameter of the wheel rim in inches, and is highly likely (on modern cars) to be anything between 14- and 22. An example, then, of a full tyre size is 225/45 R17, and you should see a similar number to this somewhere on the face of your tyres. Once you know what it is, then you can read the chart and know the correct psi/bar for your car’s tyres.

How do I top my tyres up?

At one of the aforementioned tyre inflation machines, at a fuel station, it’s simplicity itself. Straightforward buttons on the machine allow you to toggle the desired pressure between the units of psi and bar, and ‘+’/‘-’ buttons then allow you to set the tyre pressure to the level you require. Sometimes, you need to insert a coin to make them work (usually €1), so make sure you have some loose change on your person. Before you start, go round each wheel on your car and remove all four dust caps on the tyres’ valves, making sure to keep the dust caps safe.

Insert the money required, if necessary, and the machine’s compressor will start up. You now have around five minutes before the machine cuts out again. Using the long black air hose attached to each machine, go to each tyre and put the black adaptor at the end of the hose onto the exposed valve of the tyre. The machine will read the pressure of the tyre, display the current pressure on its digital screen, and then (if necessary) inflate or deflate it (if the tyre is over-inflated) to the number you pre-set in the machine. Once each tyre has reached the set pressure, or if the tyres are already at the correct pressure anyway, the machine will beep loudly two or three times, and you can move onto the next tyre, until all four are done. Replace the dust caps on the valves and you’re good to go.

Alternatively, you can use a compressor unit that plugs into your car’s 12-volt socket (these used to house cigarette lighters, but most cars still have them under ‘12V’ caps in the cabin and/or boot area) and then attaches to the valve on each tyre. This smaller unit works in a similar way to the big machines at fuel stations, but instead now there’s an ‘on/off’ switch that runs the compressor permanently and, usually, an analogue pressure gauge that shows a range of pressures in psi and bar. It’s therefore simply a case of flicking the switch and inflating the tyre until the gauge shows the right pressure for the wheel in question, so these systems aren’t as useful if you need to let air out of an over-inflated tyre.

For those with old-fashioned foot pumps, good luck! It’s a lot of effort to pump a tyre with one of these, but in essence they work on the same principle as the 12V compressor mentioned above – there’s a gauge for pressure and an attachment for the tyre valve, and all you have to do is keep pumping the metal lever, with a bit of legwork, until the gauge shows the right temperature.

How often should I check my tyre pressures?

Ideally, once a week, but that’s not always possible. Definitely check them before you head off on a long road trip or two-week driving holiday with all the family and luggage in tow, and also check them in extremely cold or hot weather, as the temperature shifts will cause air in tyres to expand/contract according to the environment. If you can’t check them once a week, try to at least do it once a month, or every other month at a push. Tyres can naturally lose a tiny bit of air over a period of time, so they will usually need topping up once or twice a year, even if they’re in perfect condition otherwise.

Finally, a lot of modern cars have tyre-pressure monitoring systems (TPMS) to warn you if they think tyres are losing air, although these can often give ‘false readings’ (i.e., the tyre isn’t actually under-/over-inflated at all, it’s rather an electrical fault with the system). However, if your TPMS warns you of a tyre issue, it’s always best to double-check that the pressures are all OK as soon as you can, and then reset the TMPS accordingly.