What tyre pressure should my car be?

Ask ten drivers what pressure their tyres should be and you will get a number, said with confidence, and it will usually be wrong. Not wrong by much, and not wrong in a way that shows up on the drive to work, but wrong in the way that quietly costs money for years. There is no standard car tyre pressure, in the same way there is no standard engine oil. The figure belongs to the car, not to tyres in general.

The good news is that your car already knows the answer and has written it down in two places. The whole job is finding the label, reading the right column, and then doing something most people never do, which is to check the tyres when they are actually cold.

This guide covers where the numbers live, why there are usually two sets, how to move between psi, bar and kPa, what under-inflation really costs, where pressure sits in the MOT, and the thing that catches out even careful drivers: the fact that a British winter takes the air out of your tyres for you without a single puncture.

Where your car’s real numbers are written down

TyreSafe puts it plainly: recommended inflation pressures for your vehicle under different operating conditions can be found in the vehicle handbook and on a placard mounted on the vehicle. In practice that placard is in one of two places on almost every car sold in the UK.

Open the driver’s door and look at the door shut, meaning the painted pillar the door closes against and the edge of the door itself. Most manufacturers put the label there, roughly at knee height. If it is not there, try behind the fuel filler flap, which is the other common spot. A few cars use the inside of the glovebox lid.

The label is small, usually white or silver, and it lists pressures by axle. It will also carry the tyre size, which is worth knowing when the time comes to price a set of new tyres.

If the car has been resprayed, or someone has replaced a door, the label can be missing. The handbook is the fallback, and every manufacturer publishes the figures. What you should not do is take a number from a forum, a tyre sidewall or another car of a similar size. The sidewall figure in particular is a trap, and it is covered further down.

Why the placard shows two sets of numbers

This is the part that gets skipped. Most placards do not give you one pressure per axle, they give you two: a set for normal everyday use, and a higher set for a fully laden car.

TyreSafe is explicit about when the second set applies. One instance where it is likely that you will need to increase your vehicle tyre pressure is when travelling with a full set of passengers or luggage, and the appropriate pressure will be listed in the handbook, door shut or filler cap.

Annex 6 of the Highway Code is unusually precise about this, and the precision is the point. It says tyres MUST be correctly inflated to the vehicle manufacturer’s specification for the load being carried. Not to a specification, to the one for the load being carried. The higher column on the placard is not a suggestion for careful drivers, it is the figure the law points at once the car is full.

The logic is simple. A tyre carries its load on a cushion of air. Put four adults and a boot full of cases into a car and the same tyre is now holding up considerably more weight on the same amount of air, so the sidewalls flex further, the tyre runs hotter and the shoulders take the strain. Raising the pressure restores the cushion.

The practical mistake is seasonal and predictable. Families set off on a summer holiday with a car loaded to the roof and the everyday pressures still in the tyres, and they do the longest, hottest, fastest drive of their year in exactly the condition the higher column exists for. If you are packing the car, use the laden figures, and drop back to the normal set afterwards. It belongs on the same list as the checks worth doing before a long drive.

Not every car offers all three, and a few offer only one. Read your own label rather than assuming the layout.

psi, bar and kPa, and how to move between them

Three units are in common use and your car may quote any of them. TyreSafe explains that either kPa, psi or bar can be used, and that it often depends on where in the world the tyres are made and whether that part of the world uses metric or imperial.

The conversions are fixed. TyreSafe gives 1 bar as 14.50 pounds per square inch, and 1 kPa as 0.145038 psi. In practice you only ever need one of them, because bar and kPa differ by a factor of one hundred.

The one rule worth remembering is that a bar figure is roughly a psi figure divided by fourteen and a half. If a label says 2.4 and a forecourt machine is set to psi, you want about 35, not 24. Setting 24 psi on a car that wanted 2.4 bar is a genuine and reasonably common error, and it leaves the tyre a third under-inflated.

What most ordinary cars actually run

These are ranges, offered so you can sanity check a number rather than replace your placard with them. Always use the label.

The number moulded into the tyre sidewall, usually alongside the word MAX, is not your pressure. It is the maximum the tyre is built to hold, which is a construction limit set by the tyre maker, not a recommendation from the people who designed your car’s suspension. Setting a tyre to its sidewall maximum will normally leave it badly over-inflated.

Cold means cold, and it means two miles

Air expands as it warms. A tyre that has been rolling along a road has flexed thousands of times a minute, and all that flexing makes heat, so the air inside it is warmer and at a higher pressure than it was on the driveway. The gauge reads the tyre as it finds it, so a warm tyre reads high.

TyreSafe sets a usable threshold: check when the tyres are cold, and before the vehicle has been driven for two miles. That is the whole rule. It also explains why the forecourt is an awkward place to do this, because getting to the forecourt is often more than two miles.

The consequence of ignoring it is the wrong way round from what people expect. If you top up a warm tyre to the placard figure, you have actually removed air, because the reading was inflated by heat. The tyre cools overnight and is now under the figure. Checking hot does not leave you slightly over, it leaves you genuinely under.

Two workarounds are practical. Either buy a gauge and a small compressor and do it on the driveway before the first trip of the day, or find the nearest air line and accept that a short, gentle drive to it adds only a small amount. What does not work is checking after a motorway run.

How to check and set them properly

The whole job takes about four minutes once you know the routine.

1. Find your figures first. Read the placard in the door shut or the filler flap and note both axles, and decide whether you are using the normal or the laden column.
2. Do it cold. First thing in the morning, or after the car has stood for a few hours. Not after a run.
3. Unscrew the valve cap and keep it somewhere it will not roll away. There is one on each wheel and they are easy to lose.
4. Press the gauge on squarely. A brief hiss as you seat it is normal, a continuous hiss means it is crooked and you are letting air out.
5. Read, then adjust. Inflate to the placard figure, or let air out in short bursts if it is over. Re-check after each adjustment.
6. Do all four, then the spare. Front and rear figures are often different, so check the label rather than copying the first wheel.
7. Refit every valve cap. The cap keeps grit and water out of the valve core, and a corroded core is a slow puncture waiting to happen.
8. Reset the monitoring system if your car asks for it, which is usually a menu item or a button in the glovebox.

While you are crouched at each wheel you are already in position for the other two-minute job, so take the coin out and check the tread at the same time. Tyres are among the most common MOT failure items and both checks are free.

The forecourt air line is the weak link

Most drivers set their pressures on a garage forecourt, using a machine that lives outdoors, gets dropped on the concrete several times a day and is calibrated by nobody in particular. TyreSafe’s advice is to use an accurate pressure gauge, which is a polite way of saying that not all of them are.

A decent pocket gauge costs a few pounds and gives you a reference. If your own gauge and the forecourt machine disagree by a psi or so, that is normal and not worth worrying about. If they disagree by five, trust your own and treat that machine as broken.

The better answer for most people is a small 12 volt compressor. It plugs into the car, lives in the boot, sets a target pressure and stops there, and it means the check happens cold on the driveway instead of warm at a petrol station. It also turns a slow puncture on a dark evening into an inconvenience rather than a callout.

What a British winter quietly takes out

This is the part almost nobody accounts for, and it is pure physics rather than anything going wrong with the car.

The pressure on your gauge is the pressure above atmospheric, and atmospheric is about 14.7 psi. So a tyre reading 33 psi actually contains air at about 47.7 psi in absolute terms. Cool that sealed volume of air down and the pressure falls in direct proportion to the temperature measured from absolute zero.

Run the arithmetic on a real British year. Set a tyre correctly to 33 psi on a mild 20C afternoon and leave it completely alone:

Nothing has leaked. No valve is faulty. The tyre has simply been left alone while the country got colder, and it has quietly given up over four psi.

Now put that next to the two figures TyreSafe publishes. An estimated 57 per cent of UK car tyres are at least 4 psi below the recommended pressures, and a tyre just 6 psi below can produce a 3 per cent increase in fuel consumption. A driver who set their pressures properly in late summer and did nothing else is already more than half way to that 6 psi mark by January, purely from the weather.

That is the practical argument for checking monthly rather than occasionally, and it is the reason a pressure check belongs in getting the car ready for winter alongside the battery and the screenwash. It is also why the warning light seems to pick cold mornings. It is not being temperamental, it is being right.

What under-inflation actually costs

Under-inflation is unusual among car faults in that it takes money off you steadily for years before it ever becomes a safety problem. TyreSafe puts numbers on it.

There is a roadside cost on top of the running cost. GOV.UK reports that one in five motorway breakdowns are caused by worn or incorrectly inflated tyres, and the same DVSA advice notes that only one in six drivers had checked their pressures before a long journey. A blowout at motorway speed is not a repair bill, it is a recovery and a lost day.

The tyre life figure is the one that stings, because it compounds. Ten per cent off the life of a set that cost you a few hundred pounds is real money, and it arrives as an early bill rather than as an obvious fault. Prices for a replacement set are set out in what new tyres cost.

There is a handling cost too. TyreSafe notes that if tyres are under or over inflated then handling and grip will worsen, potentially causing irregular or unpredictable vehicle behaviour, and that a correctly inflated tyre is less likely to aquaplane. A soft tyre also runs hot, and heat is what destroys a tyre from the inside.

Over-inflation is not the safe side

Because under-inflation gets all the attention, a fair number of drivers deliberately run a few psi high, on the theory that it saves fuel and cannot do any harm. It does do harm.

TyreSafe is direct about it: over-inflation will affect both handling and braking capabilities and results in an uncomfortable ride, and the reduced contact area with the road accelerates wear on the tread centre, meaning tyres will need replacing more quickly.

An over-inflated tyre crowns slightly, so instead of the full tread width pressing on the road you are driving on a strip up the middle. Less rubber in contact means less grip for braking and cornering, and the strip wears out while the shoulders still look almost new. The ride gets sharper over potholes too, and that transmits straight into the suspension components that were meant to be absorbing it.

Read the wear and you can see the pressure

Tyres keep a record. Run your hand across the tread and the pattern tells you what the pressure has been doing, which is useful on a used car where you have no history.

The distinction in the first three rows is worth holding on to, because it decides who fixes it. Even wear across both shoulders is a free fix with an air line. One shoulder is a job for a garage with an alignment rig, and no amount of air will cure it.

Where pressure sits in the MOT

Pressure appears in the MOT twice, in two different severities, and the difference matters.

Section 5.2.3 of the DVSA inspection manual lists a tyre obviously under-inflated as a Minor defect. A Minor is not a fail. The car passes and the item is recorded on the certificate, in the same family as the other things a tester notes but does not refuse you for. It is worth understanding what that means, which is covered in what an MOT advisory is.

The same section lists a tyre pressure monitoring system malfunctioning or obviously not working as a Major defect, and that applies to M1 vehicles first used on or after 1 January 2012, which is most cars on the road. A Major is a fail. So the warning light on your dashboard can cost you a certificate even when the tyres themselves are perfectly inflated.

Also in 5.2.3, a tyre incorrectly seated on the wheel rim is a Major, and tread depth not meeting the requirement is graded Dangerous. Tyres are one of the most common reasons cars fail, which is why they head the list in preparing for your MOT and appear again in why cars fail. For the full inspection, see what is checked in an MOT test.

The warning light, and why it comes back

The tyre pressure symbol is a horseshoe shape with an exclamation mark in it, meant to look like a cross-section of a flat tyre. It sits alongside the other dashboard warning lights, and it is one of the few that genuinely wants action rather than a booking.

There are two kinds of system underneath it. A direct system has a battery-powered sensor inside each wheel, reporting real pressure. An indirect system has no sensors at all and infers a soft tyre from the fact that it is rotating slightly faster than the others, using the ABS wheel speed sensors.

That difference explains most of the odd behaviour. An indirect system has to be told what normal looks like, so it needs resetting after you inflate the tyres or it will keep complaining. It is also blind to all four tyres going down together, which is exactly what a cold snap does. A direct system is more accurate, but its sensors have batteries sealed inside them that last roughly five to ten years, so on an older car a light that will not clear is often a dead sensor rather than a soft tyre.

Either way, the order is the same. Check all four pressures cold with a gauge first, correct them, then reset. If the light returns with the pressures verified correct, the system itself needs looking at, and it is an MOT failure item, so it is worth sorting before the test rather than after.

The spare nobody checks

If your car still has a spare, it has been losing pressure quietly since the day it was built, and nobody is checking it for you. The MOT does not inspect it. A service may or may not, depending on the schedule, and it is worth asking, as covered in what is included in a full service.

A space saver is the one to watch, because its pressure is nothing like the others. These narrow temporary wheels are commonly inflated to around 60 psi, and the figure is printed on the wheel or the tyre itself rather than on the door placard. A space saver at 33 psi is not usable.

Check it when you do the others, twice a year at minimum. A flat spare is only discovered at the exact moment it was supposed to help.

Nitrogen, and whether it is worth the money

Tyre bays sometimes offer a nitrogen fill at a few pounds a wheel. The argument is that nitrogen molecules migrate through rubber more slowly than oxygen, so pressures hold steadier, and that dry nitrogen avoids the moisture that ordinary compressed air can carry.

Both points are true and neither is large. Ordinary air is already about 78 per cent nitrogen, so you are adjusting the last fifth rather than changing the substance in the tyre. The practical objection is more telling: the moment you top up at a normal air line, you have diluted it, so the benefit only holds if you commit to going back to the same place.

Nitrogen also does nothing about temperature. The table above applies to any gas, so a nitrogen-filled tyre still loses over three psi between a mild afternoon and a frosty morning. If it is free with a set of tyres, take it. If a monthly check is the alternative, the monthly check wins comfortably.

Make it a monthly two minutes

The recommendation from TyreSafe is at least once a month and before any long journey, cold, with an accurate gauge. Annex 6 of the Highway Code is stricter still and puts it as a legal duty rather than advice, and its stated interval is weekly. That is a small ask for something that touches fuel economy, tyre life, braking distance, grip in the wet and your MOT certificate all at once.

Those two intervals differ, and it is worth being straight about it. The Highway Code says weekly, TyreSafe says at least monthly. Weekly is the counsel of perfection and monthly is what people will actually keep doing, so monthly with a gauge you trust beats weekly in theory and never in practice.

Pair it with the tread check and you have covered the only part of your car that actually touches the road, in about four minutes, with a coin and a gauge. If you would rather it was done for you, tyre pressures are part of any competent service, and asking what a garage covers is one of the questions worth asking before you book. Choosing somewhere you trust is the other half of the job, covered in how to choose a good local garage.

If the check turns up a tyre that is low every month, that is a slow puncture rather than a habit, and it wants a garage. And if you are not sure when your MOT is due, the free MOT check will tell you in a few seconds, then you can compare local garages and book in the same place. What an MOT should cost is set out in how much an MOT is.