In short. The right pressure is your car maker's figure for your car, your tyre size and your load, not the number moulded on the tyre. It is on a label in the door frame or the fuel flap, or in the handbook. The Highway Code says tyres must be inflated to that specification for the load carried, and to check them weekly, before the journey, when they are cold. A tyre that has just been driven reads about 0.3 bar high on the French makers' association's figure, and a 10 degree fall in air temperature takes about 0.11 to 0.13 bar off. The warning light is a backstop, not a gauge: the UN rule lets it wait until a tyre is a fifth down. And a bigger wheel does not set the pressure, the tyre fitted to it does: one maker's own table asks for 0.1 bar more on each step up in wheel size, another's does not rise at all.
The pressure that counts
Your tyre has two pressures that people confuse. The one moulded on the sidewall is the tyre's maximum, set by the tyre maker. The one you inflate to is your car maker's, worked out for your car's weight, your tyre size and how you use it (Pirelli UK). Pirelli adds that the maximum usually exceeds the recommended pressure.
Where to find it:
- A label on the driver's door frame or pillar.
- Inside the fuel filler flap.
- The handbook, which often has a table with several tyre sizes.
Continental adds that you will probably see two figures, one for normal use and one for full loads, and that front and rear can differ (Continental UK). If you cannot find it, Continental's advice is to ask your vehicle retailer, the manufacturer or a qualified tyre professional.
The law points at the same figure. Annex 6 of the Highway Code says tyres MUST be correctly inflated to the vehicle manufacturer's specification for the load being carried, and to refer to the handbook or data (Highway Code, Annex 6). The underlying rule is regulation 27 of the Construction and Use Regulations, which bars use of a car on a road if a tyre is not so inflated as to make it fit for the use to which the vehicle is being put (regulation 27(1)(b)).
Bar, kPa and psi
Labels use all three units. One bar is exactly 100 kPa, and one psi is 6.894757 kPa (NIST), so one bar is 14.504 psi. Common figures:
| Bar | kPa | psi |
|---|---|---|
| 1.8 | 180 | 26.1 |
| 2.0 | 200 | 29.0 |
| 2.2 | 220 | 31.9 |
| 2.4 | 240 | 34.8 |
| 2.5 | 250 | 36.3 |
| 2.7 | 270 | 39.2 |
| 2.9 | 290 | 42.1 |
| 3.2 | 320 | 46.4 |
| 3.5 | 350 | 50.8 |
Gauges are marked in one or more of them. Read the unit before you read the number: 2.2 bar and 22 psi are very different things.
How often: weekly, monthly or every fortnight?
The sources do not agree, and the difference matters.
| Where the interval comes from | What it says |
|---|---|
| Highway Code, Annex 6 | Check weekly, before your journey, when tyres are cold |
| Tyre makers' pages, UK and international | Once a month; every two to four weeks (both on one page); every two weeks in winter |
| A tyre maker's technical data book | Every 14 days, on the cold tyre, spare included |
| US federal rule 49 CFR 571.138 | The owner's manual of a new car must say each tyre, spare included, should be checked monthly when cold |
The Highway Code is the one written for British drivers. Is weekly too often? A tyre loses a little air even when it is sound: Michelin puts it at about 0.07 bar a month (Michelin France, in French) and the French tyre makers' association at up to 0.1 bar a month (TNPF, in French). That is about 0.02 bar, a third of a psi, in a week. So a weekly check is not about natural loss. It is about catching a leak, a pothole bruise or a failing valve early, while the tyre has not yet run low.
The same figures tell you how long natural loss alone takes to reach the warning light's trigger point of 20 per cent below the label:
| Label pressure (bar) | 20 per cent of it (bar) | Months at 0.07 bar a month | Months at 0.1 bar a month |
|---|---|---|---|
| 2.2 | 0.44 | 6.3 | 4.4 |
| 2.5 | 0.50 | 7.1 | 5.0 |
| 2.9 | 0.58 | 8.3 | 5.8 |
This is our arithmetic, not a measurement on a car. It says a tyre that fell by that much in a few weeks was not losing air naturally. The French association advises having tyre, wheel and valve checked by a professional once the loss reaches 0.3 bar (TNPF, in French).
Cold, warm and the weather
The UN rule on tyre pressure monitoring defines the cold pressure as the tyre pressure at ambient temperature, with no build-up from use, and the in-service pressure as the cold pressure raised by temperature effects during driving (UN Regulation 141, 2.8 to 2.10). The label's figure is a cold figure.
A driven tyre reads higher. The French association says to add 0.3 bar to the label's figure if you must read warm tyres (TNPF, in French), and the Highway Code warns that warm or hot tyres may give a misleading reading. Weather does the same in the other direction. Continental gives a rule of thumb of 0.07 to 0.14 bar (1 to 2 psi) lost for every 10 degrees C of cooling (Continental). Our calculation with the gas law lands inside that range:
| Label pressure (bar) | Per 10 °C | 20 °C colder | 20 °C warmer | 30 °C warmer |
|---|---|---|---|---|
| 2.2 | 0.11 bar (1.6 psi) | −0.22 bar | +0.22 bar | +0.33 bar |
| 2.5 | 0.12 bar (1.7 psi) | −0.24 bar | +0.24 bar | +0.36 bar |
| 2.9 | 0.13 bar (1.9 psi) | −0.27 bar | +0.27 bar | +0.40 bar |
Assumptions: the air volume stays constant, pressures are gauge pressures with 1.013 bar of atmosphere added for the calculation, and the starting point is 20 °C. A real tyre expands a little, so treat the figures as close, not exact. The practical point is the one drivers meet in October: a tyre set to the label in a warm garage reads about 0.2 bar low on a frosty morning. Continental and Transport Canada both note that large temperature changes can bring the warning light on, particularly on cold mornings. Compare tyres with each other, too: if one is 0.3 bar below the other three and they all sat in the same place, that one is losing air.
The warning light: what it can and cannot tell you
A car's pressure monitoring system (TPMS) is of one of two kinds: one uses the ABS sensors to read the wheels' rotational speeds, since an under-inflated tyre turns slightly faster; the other has a pressure sensor in each wheel (Transport Canada). The UN rule that sets its performance covers cars up to 3,500 kg and requires the light to come on within 10 minutes of driving after one tyre falls 20 per cent, or to 150 kPa (1.5 bar), whichever is higher, and within 60 minutes when up to four tyres are low together. The system must work from 40 km/h up (UN Regulation 141, 5.1.3, 5.2.1, 5.3.1). The test starts from the warm pressure after 20 minutes at around 80 km/h. Because a warm tyre reads higher than its cold figure, a system that meets the rule has lit the light by the time a tyre is 20 per cent under the label's cold figure, after the driving time the rule allows. We read the 20 per cent as a fall in gauge pressure, as tyre pressures are quoted; the regulation does not spell it out.
Here is what that means for six label pressures, with the US rule alongside (25 per cent under the label, or a minimum activation pressure, whichever is higher) (49 CFR 571.138):
| Label pressure (bar) | UN rule: light on at or before (bar) | In psi | US rule: 25 per cent under (bar) |
|---|---|---|---|
| 1.8 | 1.50 (150 kPa floor) | 21.8 | 1.35 |
| 2.0 | 1.60 | 23.2 | 1.50 |
| 2.2 | 1.76 | 25.5 | 1.65 |
| 2.5 | 2.00 | 29.0 | 1.88 |
| 2.9 | 2.32 | 33.6 | 2.18 |
| 3.2 | 2.56 | 37.1 | 2.40 |
These are the latest points the rules allow, not what your car does: it may warn earlier. But a tyre at 2.0 bar on a 2.5 bar label has not set off the light yet, and is already 20 per cent down. Continental says a TPMS does not replace the check, and Pirelli calls a pressure 25 per cent below the recommended one severely low (Continental UK, Pirelli UK).
After you correct a pressure, the system may need resetting; the UN test ends with inflating to the label and resetting as the maker says (annex 3, 2.8).
In the MOT, as the Driver and Vehicle Standards Agency (DVSA) sets it out for testers, the TPMS inspection applies to cars first used on or after 1 January 2012. A TPMS that is malfunctioning or obviously not working is a major defect, and a tyre obviously under-inflated is a minor one. A lamp that stays on after an earlier pressure loss is not automatically a malfunction, and a spare tyre is not inspected (DVSA MOT inspection manual, 5.2.3). We go through that in MOT, TPMS and aftermarket wheels.
What changes with bigger wheels
Start with what does not change. The wheel's material or how it was made does not set the pressure: a forged wheel and a cast one of the same size on the same car take the same figure. What sets it is the tyre fitted on it, its load capacity and the load on the axle.
What does change as the diameter goes up with the same tyre width and a similar overall diameter is the sidewall, and with it the air. For a 225 mm wide tyre, our arithmetic gives:
| Size | Sidewall height | Air section (cm²) | Against 17 inch |
|---|---|---|---|
| 225/45 R17 | 101 mm | 1,696 | 100% |
| 225/40 R18 | 90 mm | 1,547 | 91% |
| 225/35 R19 | 79 mm | 1,389 | 82% |
| 225/30 R20 | 68 mm | 1,220 | 72% |
The air section is the ring between the rim and the tyre's outside diameter. It ignores the thickness of the tyre wall and the shape of the rim, so it shows the trend, not the litres. The trend is clear: each step up holds about a tenth less air. With less air, the same loss shows up as a larger fall in pressure, and there is less rubber between the rim and a kerb or a pothole. That is a reason to check more carefully, not a reason for a different figure.
The figure itself is in the maker's tables, size by size, and the tables do not agree on a rule:
| Car, from its maker's table | Tyre | Axle | Part load (bar) | Full load (bar) |
|---|---|---|---|---|
| Porsche 911 (992) | 235/40 ZR 19 | front | 2.2 | 2.4 |
| Porsche 911 (992) | 245/35 ZR 20 | front | 2.3 | 2.5 |
| Porsche 911 (992) | 295/35 ZR 20 | rear | 2.5 | 3.0 |
| Porsche 911 (992) | 305/30 ZR 21 | rear | 2.6 | 3.1 |
Source: Porsche bulletin 38/25, page 14, cold tyres at about 20 °C. Moving up one size adds 0.1 bar at either axle here. Porsche's table has two more columns for its comfort air pressure setting, and prints psi values too, which we leave out. Now another car's table:
| MINI Clubman (Cooper SD), from Michelin UK | Position | Pressure listed |
|---|---|---|
| 175/60 R16 82H | front and rear | 2.6 |
| 195/55 R16 87V | front and rear | 2.4 |
| 205/45 R17 84V | front and rear | 2.6 |
| 205/40 R18 82W | front and rear | 2.6 |
The page prints no unit; the values fit bar. The 16 inch tyre with the lower load index asks for more than the 16 inch tyre with the higher one, and the 17 and 18 inch sizes are equal. So there is no rule such as "one inch bigger, add 0.1 bar". Michelin's own note under the table tells the reader that a tyre dealer can say whether the pressure should be adjusted for an alternative size.
What a tyre can carry depends on its pressure, and that is why the figure is tied to the tyre and not to the wheel. Continental's technical data book (2015 to 2016 edition, values based on the European tyre standard) tabulates load capacity against pressure for every load index. For index 91, the index of a common 205/55 R16:
| Pressure (bar) | Load capacity of an index 91 tyre | Share of its 2.5 bar rating |
|---|---|---|
| 2.0 | 515 kg | 84% |
| 2.1 | 535 kg | 87% |
| 2.2 | 555 kg | 90% |
| 2.3 | 575 kg | 93% |
| 2.4 | 595 kg | 97% |
| 2.5 | 615 kg | 100% |
Source: Continental technical data book, table 1 (standard load tyres). The rows are linear enough that each 0.1 bar is worth about 20 kg on this tyre. At 2.0 bar, the point where a 2.5 bar car would be 20 per cent down, the tyre has given up about a sixth of its rated load. The same table shows why a tyre with a higher index can run at a lower pressure: 475 kg is reached at 2.5 bar by an index 82 tyre and at 2.1 bar by an index 87 tyre, the two indices of the MINI table above. The direction is the same as in that table; the maker's figure also reflects the axle load, comfort and handling, so the numbers do not match one for one. Reinforced (XL) tyres have their own table, and reach a given rating at a higher pressure: an index 94 tyre carries 670 kg at 2.5 bar in the standard-load table (table 1) and at 2.9 bar in the reinforced one (table 2). The book also says the tabulated pressures are minimums for speeds up to 160 km/h (100 mph), that they may be increased on the car maker's advice, and that the pressure of warm tyres should never be reduced, because it could fall below the minimum once they cool (data book, page 107).
Keep the load index at or above the car's original: Continental's advice is that you can go higher but never lower (Continental UK), and our guide to reading a tyre size explains the markings.
So, when you change the size:
- If your new size is in the car's own table, use that line. Nothing else is needed.
- If it is not, ask the tyre fitter or the tyre maker for the new tyre's load and inflation data, and set it against the load on your axle. The US federal rule makes a car's owner's manual say that, for a tyre of a different size from the label's, you should determine the proper inflation pressure for it (49 CFR 571.138). The rule does not say how; the answer comes from the tyre's load table and the load.
- Do not size a pressure from the wheel's diameter alone, and do not use the sidewall figure.
- Ask for the sensors to be matched to the new wheels and the system reset. In the UN test, a wheel or tyre incompatible with the system is one way to simulate a malfunction.
Our what size wheels fit my car guide explains how a tyre size is read against a car, and the tyre size calculator compares two sizes. Neither gives a pressure, because neither can know your axle load.
A weekly routine that takes five minutes
- Check before the journey, when the car has stood for a few hours, in the same place each time.
- Use your own gauge if you can; forecourt gauges can be inaccurate, as Pirelli notes. Read the unit.
- Read the label's figure for your tyre size, and the right column for your load.
- Check all four tyres, and the spare if there is one.
- Add air to the label figure, or let a little out; read again.
- Refit the valve caps; a missing one lets grit into the valve.
- If one tyre keeps needing air, it is leaking: read slow puncture: causes and fixes.
- Reset the pressure warning if your car asks for it.
Wheels from Onyx Forged
We do not publish a pressure for your car, and we do not give one for a size: the figure is your car maker's and your tyre's, as above. The sensors, their programming and the fitting are done by your tyre fitter. Our warranty does not cover tyre pressures outside the car maker's recommendations (article 6.2 of the terms and conditions). If you are unsure about a tyre size for made-to-order wheels, ask customer service before the tyres are fitted, not after.
Frequently asked questions
What should my tyre pressure be?
The figure your car maker gives for your car, your tyre size and your load: on a label in the door frame or fuel flap, or in the handbook. The Highway Code says tyres must be inflated to the maker's specification for the load being carried. There is no one figure for every car.
Is the pressure on the tyre sidewall the one to use?
No. It is the tyre's maximum, not what your car needs. Pirelli and Continental both say the maker's recommended figure is the one to follow.
How often should I check my tyre pressure?
The Highway Code says weekly, before a journey, when the tyres are cold. Tyre makers' pages say once a month or every two to four weeks. A weekly check is about spotting a leak early: natural loss is about 0.02 bar a week.
Should I check tyre pressure when the tyres are warm or cold?
Cold, which means before driving or after the car has stood for some hours. The Highway Code says warm or hot tyres may give a misleading reading. If you must read warm tyres, the French tyre makers' association adds 0.3 bar to the label's cold figure.
Why does my tyre pressure warning light come on when it gets cold?
Pressure falls about 0.11 to 0.13 bar for every 10 degrees C of cooling, so a tyre set to the label in a warm place can be well under on a frosty morning. Check with a gauge and top up to the label figure.
Does tyre pressure change with bigger wheels?
Not because of the wheel. It depends on the tyre fitted, its load capacity and the load on the axle. Porsche's table for the 911 asks for 0.1 bar more on each step up in wheel size; MINI's table for the Clubman does not follow wheel size at all. Use the maker's table for your tyre size, or ask a fitter for the tyre's load and inflation data.
Do I still need to check the pressure if my car has a warning light?
Yes. The UN rule lets the light wait until one tyre is 20 per cent down, or 150 kPa, whichever is higher. At a 2.5 bar label that is 2.0 bar.
What pressure should the spare wheel have?
The one on its label or in the handbook. A T-type space saver's tyre is marked INFLATE TO 420 kPa (60 psi); our spare wheel guide covers it.
Where to go next
If you change a wheel yourself, read how to change a wheel safely; if a tyre keeps losing air, slow puncture: causes and fixes. Onyx Forged is a French brand of made-to-order forged wheels, designed to JWL/VIA standards: see Onyx Forged and the catalogue.
Sources
- GOV.UK, The Highway Code, Annex 6: vehicle maintenance, safety and security (updated 22 October 2025).
- legislation.gov.uk, The Road Vehicles (Construction and Use) Regulations 1986, regulation 27.
- GOV.UK, MOT inspection manual, section 5.2.3: tyres and the tyre pressure monitoring system.
- UN Regulation No. 141, tyre pressure monitoring systems (Official Journal L 269, 26 October 2018).
- 49 CFR 571.138, Standard No. 138, tire pressure monitoring systems (US).
- Porsche AfterSales, bulletin 911 (992) 38/25, wheels and tyres list (7 October 2025), page 14.
- Michelin UK, MINI Clubman tyre pressures and sizes.
- Continental, Tyre pressure (UK); Continental, Load index (UK); Continental, Tire pressure in winter; Continental, Technical Data Car, 4x4, Van 2015-2016.
- Michelin (Middle East), Tyre pressure for cars; Pirelli UK, How to check tyre pressure.
- Pages in French: TNPF, checking the inflation pressure; TNPF, advice on inflation pressures; Michelin France, which pressure for your car's tyres.
- Transport Canada, Tire pressure monitors.
- NIST Special Publication 811, Appendix B.9, conversion factors.
All pages were read on 15 October 2026.
The Onyx Forged design office.