Guides
Tyre pressure — finding the right figure and keeping it
The pressure your car needs is printed on the car, not on the tyre. Where to find the figure, how to check it on cold tyres, what psi and bar mean — and what the TPMS light is actually telling you.
Last reviewed: September 2026 · LCDrive editorial team
Where the correct figure lives
Every car has a set of recommended pressures chosen by its manufacturer, and finding them takes seconds: they are in the vehicle handbook, on a sticker in the driver's door opening — on the sill or the frame — or inside the fuel filler flap, depending on the make. Front and rear figures often differ, and the placard often lists them by tyre size, so read the row that matches the size on your sidewall. If you are not sure which size that is, look your car up by registration and the right row is easy to spot.
The number moulded into the tyre sidewall is not the recommendation. It is the maximum pressure the tyre itself can safely hold, set by the tyre maker with no knowledge of the car it would end up on — never inflate to it. The figure to use is the car's, and that is a legal point as well as a technical one: the Highway Code says tyres must be correctly inflated to the vehicle manufacturer's specification for the load being carried, a requirement that comes from the Construction and Use Regulations.
PSI and bar — reading the numbers
Handbooks, placards and forecourt machines mix two units. Many UK placards quote psi (pounds per square inch), many forecourt machines are set in bar, and plenty of stickers give both. The relationship is fixed — 1 bar is 14.5 psi — so a figure in one unit always has a direct equivalent in the other. The table below is a conversion aid, not a set of recommendations: take the figure from your placard and read across.
| Pounds per square inch | Bar |
|---|---|
| 26 psi | 1.8 bar |
| 30 psi | 2.1 bar |
| 33 psi | 2.3 bar |
| 36 psi | 2.5 bar |
| 40 psi | 2.8 bar |
How to check — cold tyres, at home or on the forecourt
Check pressures cold — meaning the car has been parked for a few hours, or driven no more than a couple of miles. Pressure rises as a tyre warms with driving, so a warm reading flatters: set warm tyres to the placard figure and they will be under-inflated by the time they cool.
At home
Unscrew the valve cap, press a gauge squarely onto the valve, and compare the reading with the placard figure for that axle. Top up with a foot pump or a small compressor, check again, and refit the valve caps — they keep grit out of the valve core.
On the forecourt
Set the target pressure on the machine before connecting the hose, using the placard figure for the axle you are on, and let the machine fill or bleed to the target. Mind the units — if the placard reads in psi and the machine is set to bar, convert first rather than guessing.
Do all four tyres, and the spare if you carry one — a flat spare discovered at the roadside is no spare at all. Check at least once a month — the Highway Code actually suggests weekly — and before any long journey. And since you are already kneeling at each wheel, it takes seconds more to check your tread depth at the same time.
Carrying more — the loaded figures
Look again at the placard and most cars show a second, higher set of pressures for a fully loaded vehicle — a full complement of passengers plus their luggage. This is the “for the load being carried” clause of the Highway Code in practice: the right pressure depends on the weight the tyres are asked to carry, and the loaded figures are typically a few psi higher than the everyday set. Use them for the holiday run, the airport trip with a full boot, the tip run with the seats folded — then drop back afterwards, because the loaded figure over-inflates an empty car.
What the wrong pressure does
Under- and over-inflation fail in different ways, and neither is benign. The patterns are distinctive enough that a worn tyre will often tell you which one it suffered.
| Effect | Under-inflated | Over-inflated |
|---|---|---|
| Tread wear | Both outer edges — the tyre runs on its shoulders | Centre of the tread — the tyre runs on its crown |
| Contact patch | Distorted, so the tread cannot clear water as designed | Shrunken — less rubber on the road, less grip |
| Running | More flex and heat, more rolling resistance | Harsher ride, more vulnerable to pothole and impact damage |
| Wet roads | More prone to aquaplaning, longer wet braking | Aquaplaning risk rises here too — more air is not safer |
Under-inflation is the slow, expensive one. A soft tyre flexes more with every rotation, which costs fuel — it can add roughly 3% to consumption — and builds heat, and sustained heat can degrade the casing until it fails suddenly, with motorway speed, long runs and heavy loads the worst combination. The distorted contact patch also stops the tread clearing water the way it was designed to, so wet braking suffers long before the tyre looks flat.
Over-inflation looks tidier but is not safer. A hard tyre balloons in the middle and runs on its crown: the contact patch shrinks, grip falls, the ride hardens, and the stretched casing is more vulnerable to pothole and impact damage. It raises aquaplaning risk too — correct pressure, not maximum pressure, is what keeps the tread working.
The TPMS light — what it is telling you
In practice, every new car sold in the UK since late 2014 has left the factory with a tyre pressure monitoring system. Two designs exist: direct systems put a pressure sensor inside each wheel, while indirect systems fit no sensors at all and infer a softening tyre from the wheel-speed data the ABS already collects. Both feed the same dashboard warning lamp.
A lamp that comes on and stays on means the system has seen a pressure loss. Treat it as exactly that: check all four tyres with a gauge as soon as you safely can, and never simply reset the light without checking — clearing the warning does not reinflate the tyre. On many cars, a lamp that flashes for a spell and then stays on is different: the system is reporting a fault with itself, often a dead sensor, rather than low pressure.
On cars with in-wheel sensors, each sensor runs on a sealed battery that typically lasts five to ten years, and when it dies the sensor is replaced whole — the battery cannot be changed. Diagnosing sensor faults, then replacing and programming sensors, is what our mobile TPMS service handles, at your home or workplace.
Either way, TPMS supplements the monthly check with a gauge — it never replaces it. Indirect systems in particular are a prompt, not a gauge: they flag a tyre going soft but measure nothing. The system earns its keep most on run-flat tyres, which depend on a working TPMS to report the puncture their stiff sidewalls are busy hiding. And if a tyre is losing pressure faster than a forecourt top-up can answer, that is what Emergency roadside help is for.
Pressure, TPMS and the MOT
TPMS has been part of the MOT since the start of 2015. For cars first used on or after 1 January 2012, the tester checks the system, and one that is malfunctioning or obviously not working is a major defect — a fail. A dead sensor is therefore not a quirk you can live with until the light stops bothering you: it is a test failure waiting for the next MOT.
Pressure itself appears too: a tyre that is “obviously under-inflated” is recorded as a minor defect — not a fail on its own, but a written note that you were told. And beyond the MOT, a seriously under-inflated tyre can be treated as a defective tyre under the Construction and Use Regulations, where the penalty can reach £2,500 and three penalty points per tyre.
If a check turns up a tyre that has been run soft for a while — shoulders worn smooth, wet grip gone — replace it rather than nurse it. You can browse replacement tyres by size and the fitting comes to you — anywhere in the UK, at home, at work or at the roadside.