In short. A tyre loses a little air even when it is sound, about 0.07 to 0.1 bar a month, and a 10 degree fall in temperature takes 0.11 to 0.13 bar off, so first take the weather out. What is left is a leak if one tyre is 0.3 bar below where its neighbours are, or keeps needing air. The air escapes in one of five places: the tread, the valve, the seat where the tyre sits on the rim, the wheel itself, or a damaged tyre. A fitter finds which with the wheel off the car, and that decides what can be done: a tread puncture of about 6 mm in the central part can be repaired from the inside; a sidewall cannot; a valve is replaced; a corroded bead seat on an aluminium wheel can be cleaned and sealed. Do not drive on it for long: the Highway Code says stop as soon as it is safe to do so, and a tyre 20 per cent down carries a sixth less load.
Is it a puncture, or the weather?
Two things change a tyre's pressure without any hole in it.
Natural loss. Rubber is not completely airtight. Michelin puts the normal loss 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, even if the car does not move (TNPF, in French). Michelin's Middle East page says there is a small amount of normal air loss whatever the age and wear of the tyre.
Temperature. Continental's rule of thumb is 0.07 to 0.14 bar (1 to 2 psi) lost for every 10 degrees C of cooling (Continental). Our gas-law calculation, in the tyre pressure guide, gives 0.11 to 0.13 bar. A 10 degree fall is therefore worth one to two months of natural loss.
That is why a single reading proves nothing. Compare the same tyre across days, take the temperature out, and compare tyres with each other. A worked example with made-up numbers:
| Tyre | Day 1 at 20 °C | Day 8 at 10 °C | Fall | Cold weather alone predicts | Left over | Our reading |
|---|---|---|---|---|---|---|
| A | 2.30 bar | 2.19 bar | 0.11 bar | 0.11 bar | 0.00 bar | The weather |
| B | 2.30 bar | 1.96 bar | 0.34 bar | 0.11 bar | 0.23 bar | A leak: book a check |
| C | 2.30 bar | 2.12 bar | 0.18 bar | 0.11 bar | 0.07 bar | Read again in a few days |
How to read it: the "Cold weather alone predicts" column assumes a 10 °C fall on a tyre at 2.30 bar, using the gas law with the air volume constant. Natural loss over a week is only about 0.02 bar, so 0.07 bar left over in a week is more than that, and 0.23 is far more. The French association's advice is to have the tyre, wheel and valve checked by a professional once the loss reaches 0.3 bar (TNPF, in French). The verdict column is our rule of thumb, not a standard. Use your own gauge, read the tyres cold, and note the date and temperature each time.
The warning light is a late signal. UN Regulation 141 lets it wait until a tyre is 20 per cent below the cold figure, or at 150 kPa (1.5 bar), whichever is higher (UN Regulation 141, 5.2.1 and 5.3.1). On a 2.5 bar label that is 2.0 bar. By then, according to Continental's data book, a tyre of index 91 can carry 515 kg instead of 615, 84 per cent of its rating (data book, table 1).
Where the air goes
| Where | What it looks like | How to confirm | What fixes it |
|---|---|---|---|
| The tread: a nail, a screw, a thorn | A ticking that speeds up with the car; an object in the tread | Look around the tread; water on the spot shows bubbles | A repair from the inside, if the hole and place allow (see below) |
| The valve | No visible damage; a missing cap lets grit in; a cracked or porous valve | Water around the valve shows bubbles | A new valve; a damaged valve can be a dangerous MOT defect |
| The bead seat: where the tyre meets the rim | A slow leak with nothing in the tread | A leak test in a water tank, tyre off; corrosion shows only when the tyre is dismounted | Clean and reseal the seat; or replace the wheel if the damage is beyond the limits |
| The wheel itself | After a kerb or pothole, a bent flange or a crack | A fitter's inspection of the rim | Replace the wheel if the flange is bent or the rim cracked |
| The tyre's structure | After an impact, no puncture visible; or a tyre driven low | Inspection from inside | Replace the tyre |
On the tread, Carbuyer notes that an object may be plugging the hole it made, so pulling it out can let the tyre deflate faster; take it to a fitter instead (Carbuyer). A Land Rover repair kit text says the same: do not try to remove nails or screws (Land Rover). On the valve, a fitter's page suggests inflating the tyre and rubbing soapy water around the valve, with bubbles confirming a leak (National Tyres).
The valve. The MOT lists a tyre valve seriously damaged or misaligned and likely to cause sudden deflation as a dangerous defect (5.2.3 (j)). BS AU 159G covers the repair of inflation valves as well as tyres. Porsche's wheel bulletin says to always use new valves when changing tyres (Porsche).
The bead seat on aluminium wheels. General Motors' bulletin 08-03-10-006I, written for its own cars and light trucks, describes a tyre that slowly loses air over days or weeks. Abrasive and corrosive matter gets between tyre and wheel at the bead seat, and there is always some relative motion between them when the car is driven, so a path for air can develop. The corrosion may be on the inboard or outboard seat and is not visible until the tyre is dismounted. The bulletin's repair: after a water-tank leak test, dismount, measure, and if the total length of corrosion is 100 mm (4 in) or less on a car with 32,186 km (20,000 miles) or less, or 200 mm (8 in) or less on one above that, clean it with a fine disc down to firm material and coat the seat with a bead sealer. Beyond those lengths, replace the wheel. The bulletin is for professional technicians and covers machined or painted aluminium wheels; it sends porosity of cast wheels to a different bulletin (General Motors). Porosity is a casting matter: see forged against cast wheels.
The wheel. EUWA, the wheel makers' association, lists among the defects that require replacing a wheel bent rim flanges, generally from impacts against obstacles or potholes, and cracks in the rim area caused by a tyre puncture. It also says a part with suspected damage from severe pothole shocks must be replaced (EUWA ES 1.07, 5.2 and 5.3). Our guide to alloy wheel repair goes into what can and cannot be done.
How long can you drive on it?
| Situation | The limit | Where it comes from |
|---|---|---|
| A flat tyre | Stop as soon as it is safe to do so; change it only if you can do it without putting yourself or others at risk, otherwise call a breakdown service | Highway Code, Annex 6 |
| The pressure warning light is on | Stop and check; the light may wait until 20 per cent or 150 kPa, and a tyre that far down carries about a sixth less load | UN Regulation 141; Continental data book |
| A slow loss you have topped up | Have it looked at soon; Carbuyer says to avoid driving until it is repaired or replaced, because a tyre driven semi-inflated may not be repairable | Carbuyer |
| A tyre sealed with a repair kit | One maker's text: up to 80 km/h (50 mph) and 200 km (125 miles); one fitter's page says no more than 30 mph | Land Rover text; fitter's page |
| A run-flat tyre that is flat | Built for at least 80 km/h (50 mph) over 80 km (50 miles) | UN Regulation 30 |
| A space saver fitted | Not above 50 mph | Construction and Use Regulations, 27(3) |
The kit and the space saver are in spare wheel, space saver or repair kit.
What a fitter can repair, and what they will not
A tyre is repaired from the inside. The standard in Britain is BS AU 159G:2013, and its scope excludes liquid sealants and temporary seals, which it describes as externally inserted plugs that let the car be driven for a limited period until a permanent repair (BSI Knowledge). One fitter's page says it does not do a "string" repair from the outside, because the technician cannot inspect the inside of the tyre (National Tyres). The standard's text is paid; these are the limits that makers and fitters publish:
| Damage | Repairable? | Basis |
|---|---|---|
| A small puncture in the central part of the tread, about 6 mm or less | Yes, if the inside is sound | Continental: central three-quarters, 6 mm; a fitter: central three-quarters, under 6 mm |
| A puncture in the sidewall or the shoulder | No | Continental: sidewall damage is never repairable; Pirelli |
| A hole of more than 6 mm | No | Continental; a fitter's page |
| A tyre driven under-inflated or flat | No, in a fitter's list: damage caused by under or over inflation | A fitter's page; Continental: structure compromised |
| A run-flat tyre | Manufacturers advise against | Pirelli; Continental: run-flat damage is not repairable |
| A valve leak or a corroded rim | The tyre is not the cause: valve or wheel is fixed | Continental |
| Exposed cords, damaged bead, cuts, solvent contamination, an earlier repair not to the standard | No | A fitter's list |
The sources do not describe the repairable zone alike. Continental and the fitter say the central three-quarters of the tread, and Pirelli says the "T area" is between 60 and 70 per cent of the nominal width. The size limit reads as 6 mm in each, though Continental's page treats a hole of exactly 6 mm both ways. The same fitter says tyres rated V and above can have one repair in their life. If you want certainty, ask which part of BS AU 159G the fitter works to.
A sealant leaves its mark: tell the fitter it was used. One Land Rover text says to make the repair centre aware before the tyre is removed, and that sealant can cause error prompts and incorrect readings from the pressure monitoring system. A fitter's page says it repairs tyres treated with water-based sealant, which can be removed from inside.
The MOT and a slow puncture
A slow puncture is not an MOT item by name. What the tester can fail is its consequences: a tyre obviously under-inflated is a minor defect, a tyre pressure monitoring system malfunctioning or obviously not working is major, and a seriously damaged or misaligned valve is dangerous (DVSA MOT manual, 5.2.3). The Highway Code adds that tyres must be correctly inflated to the vehicle manufacturer's specification for the load being carried, and that under or over inflated tyres adversely affect brakes and steering. The pressure monitoring system and aftermarket wheels are in MOT, TPMS and aftermarket wheels.
Bigger wheels and slow punctures
A taller wheel with a lower tyre has less rubber between the rim and the road. A fitter's page notes that a kerb strike lands on the sidewall, which is much thinner than the tread, and that the valve, being on the side, may be damaged in the same impact. That is why a low-profile set deserves a second look after any hard knock: check the pressure over the next days, as in the table above, and look at the rim and the valve.
When new tyres go on new wheels, ask for new valves, as Porsche's bulletin says, and ask whether the bead seats were looked at when the tyres went on. The pressure monitoring sensors, if your car has them in the valves, are fitted by the same fitter; the details are in the TPMS article linked above.
Wheels from Onyx Forged
Finding a leak at the bead or the valve of any wheel takes a tyre fitter with the tyre off the wheel; go back to the fitter first. If a leak persists on a made-to-order wheel after the fitter has checked the valve and the tyre, write to customer service with what the fitter found. The Onyx Forged design office does not give a repair or a pressure for your car: those are the fitter's and the car maker's.
Frequently asked questions
What causes a slow puncture?
A nail, screw or thorn in the tread; a leaking valve; corrosion or damage at the bead seat of the wheel; a bent or cracked wheel; or a tyre damaged by an impact or by driving it low. A fitter finds which with the wheel off the car.
How do I know if I have a slow puncture?
One tyre needs air more often than the others, or is 0.3 bar below where its neighbours are after the same weeks, and cold weather does not explain it. The car may pull to one side. Use the table above: measure cold, with your own gauge, and take the temperature out.
How long can you drive on a slow puncture?
Not far. The Highway Code says to stop as soon as it is safe to do so when a tyre is flat, and Carbuyer advises avoiding driving until the tyre is repaired or replaced. At 20 per cent below the label a tyre carries about a sixth less load.
Can a slow puncture be repaired?
If it is in the central part of the tread and about 6 mm or less, usually yes, from the inside. Sidewall damage, a tyre driven flat and run-flat damage are not repaired. If the leak is the valve or the rim, the tyre is not the problem.
Can I fix a slow puncture myself?
A repair kit seals a tread puncture temporarily, within the maker's limits, and then the tyre should go to a fitter. A string plug from the outside is a temporary seal that BS AU 159G does not cover, and one fitter does not do it.
Is a slow puncture an MOT fail?
Not by name. A tyre obviously under-inflated is a minor defect, a failed pressure monitoring system is major, and a seriously damaged valve is dangerous.
Why does my tyre lose pressure when it is cold?
Because the air contracts: about 0.11 to 0.13 bar for every 10 degrees C. Top up to the label figure when cold and check again in a few days; if only one tyre keeps falling, it is leaking.
Can an alloy wheel cause a slow puncture?
Yes. Corrosion at the bead seat of an aluminium wheel can open a path for air; a bent flange or a crack can too. A wheel with a leak at the seat can sometimes be cleaned and sealed, within the maker's limits, and a bent or cracked wheel is replaced.
Where to go next
For the pressure itself, read tyre pressure: what it should be and when to check it; for a change at the roadside, how to change a wheel safely; for a vibration after the repair, wheel balancing. 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).
- GOV.UK, MOT inspection manual, section 5.2.3.
- UN Regulation No. 141, tyre pressure monitoring systems; UN Regulation No. 30, pneumatic tyres; The Road Vehicles (Construction and Use) Regulations 1986, regulation 27.
- BSI Knowledge, BS AU 159G:2013 (the paid text is not read; the scope is).
- Continental, Tire repair; Continental, Tire pressure in winter; Continental, Technical Data Car, 4x4, Van 2015-2016.
- Pirelli UK, Tyre puncture repair; National Tyres, Tyre puncture repairs.
- General Motors, bulletin 08-03-10-006I (March 2025); Porsche AfterSales, bulletin 911 (992) 38/25 (7 October 2025).
- Carbuyer, How to spot a slow puncture; Michelin (Middle East), Tyre pressure for cars; Land Rover, owner's information, Tyre repair kit.
- EUWA, ES 1.07, Safety and Service Instructions for the use of Wheels in Tyre Workshops (June 2020), section 5.
- Pages in French: TNPF, checking the inflation pressure; TNPF, advice on inflation pressures; Michelin France, which pressure for your car's tyres.
- NIST Special Publication 811, Appendix B.9, conversion factors.
All pages were read on 15 October 2026.
The Onyx Forged design office.