In short. Nitrogen does keep a tyre at its pressure slightly longer than air, but the difference is small, and it changes nothing else you would notice on a road car. In Consumer Reports' year-long test, tyres filled with nitrogen lost 2.2 psi on average and tyres filled with air lost 3.5 psi, a gap of 1.3 psi over twelve months; a 20 °C drop in temperature takes about 3.5 psi out of either. The US tyre makers' association says nitrogen inflation is "not necessary" for normal service, Continental says the same for everyday driving, and Michelin says its tyres perform as designed with either gas. Where nitrogen does matter is in aircraft, heavy and mining equipment, and racing: large aeroplanes must, by US rule, run braked-wheel tyres with no more than 5 per cent oxygen.
What "nitrogen" actually means in a tyre
Air is already mostly nitrogen. Consumer Reports gives dry air as 78 per cent nitrogen, 21 per cent oxygen and 1 per cent other gases, with almost always some water vapour as well. The US Tire Manufacturers Association (USTMA) calls nitrogen "basically, nothing more than dry air with oxygen removed" (TISB 44, June 2024), and Michelin US puts it the same way (Michelin US, tire pressure).
So a "nitrogen fill" does two things: it raises the share of nitrogen from 78 per cent to whatever the machine can deliver, and it removes the water. The generator Consumer Reports used claimed 95 per cent purity and removes water vapour from the ambient air it processes. Those two changes are the whole story. Every claimed benefit comes from less oxygen, less water, or both.
What the tyre makers say
| Source | What it says about nitrogen in car tyres |
|---|---|
| USTMA, TISB 44 (June 2024) | "For normal tire service applications, nitrogen inflation is not necessary." It is permissible, as it "may contribute to minor reductions in inflation pressure loss", but punctures, the bead, the valve and the wheel can negate that benefit. Nitrogen and air "can be mixed in any and all proportions." |
| Michelin US (updated 28 April 2026) | Because nitrogen replaces oxygen, less escapes and pressure stays higher longer; other leak paths "prevent the guarantee of pressure maintenance" with either gas. Nitrogen and air "CAN be mixed". Michelin tyres perform as designed with air or nitrogen at the vehicle maker's pressure. |
| Continental (Australian page) | Not necessary for typical everyday use on a passenger car, and "not harmful". Air-filled tyres lose pressure through permeation a little more quickly, but there is "no discernible benefit" for rolling resistance, fuel economy or tyre ageing. |
The three agree on the facts and on the conclusion. None of them tells you to avoid nitrogen; all of them say it does not replace checking the pressure. The USTMA goes furthest: depending on nitrogen alone to reduce inflation maintenance "may lead to underinflated operation", which can cause premature tyre failure.
The test results: how much pressure nitrogen saves
Consumer Reports ran a 12-month study at its test centre with 31 models of all-season tyre. One tyre of each model was filled with air and another with nitrogen at 95 per cent purity, both to 30 psi at room temperature. The tyres sat outdoors for a year and were measured again at room temperature. Air-filled tyres lost 3.5 psi on average; nitrogen-filled tyres lost 2.2 psi (Consumer Reports, updated 11 September 2026).
Put on a monthly scale, which is our arithmetic, that is 0.29 psi a month for air against 0.18 psi for nitrogen: a saving of about 0.11 psi a month, or 0.09 bar over the year. Michelin US says tyres "have been known to lose up to 1psi" a month. The nitrogen tyres in the test therefore still leaked, just a little more slowly. The tyres were mounted on wheels and left standing, so both gases faced the same bead, valve and wheel leaks, which the USTMA and Michelin say nitrogen does nothing for; punctures and knocks from driving were not part of the test.
Consumer Reports also reports what the US National Highway Traffic Safety Administration (NHTSA) found in its own laboratory study: air escapes faster than pure nitrogen, nitrogen reduces the ageing of the rubber by limiting oxidation, and inflating with nitrogen "had little or no direct effect on tire rolling resistance performance". We could not read the NHTSA report itself (its server refused our request), so these findings are given here as Consumer Reports quotes them. On ageing, Consumer Reports adds that it is not a practical concern for most drivers, who replace tyres through wear long before oxidation matters. How age and the DOT date code limit a tyre's life, whatever gas it holds, is covered in how long tyres last.
The gas law: why cold mornings matter more than the gas
The pressure in a tyre follows the ideal gas law. At a fixed volume, the absolute pressure rises and falls in proportion to the absolute temperature. The law has no term for the kind of gas: dry nitrogen and dry air change by the same amount for the same temperature change. This table is our calculation, with a constant volume, gauge pressure plus 1.013 bar of atmosphere, and a start at 20 °C:
| Pressure set at 20 °C | At −10 °C | At 0 °C | At 40 °C | At 60 °C | At 80 °C |
|---|---|---|---|---|---|
| 2.2 bar (32 psi) | 1.87 bar (−4.8 psi) | 1.98 bar (−3.2 psi) | 2.42 bar (+3.2 psi) | 2.64 bar (+6.4 psi) | 2.86 bar (+9.5 psi) |
| 2.5 bar (36 psi) | 2.14 bar (−5.2 psi) | 2.26 bar (−3.5 psi) | 2.74 bar (+3.5 psi) | 2.98 bar (+7.0 psi) | 3.22 bar (+10.4 psi) |
| 2.9 bar (42 psi) | 2.50 bar (−5.8 psi) | 2.63 bar (−3.9 psi) | 3.17 bar (+3.9 psi) | 3.43 bar (+7.7 psi) | 3.70 bar (+11.6 psi) |
A real tyre stretches a little, so treat these figures as close, not exact. They match the common rule of 1 psi for every 10 °F: 10 °F of difference is 5.6 °C, and our calculation gives 0.97 psi for 5.6 °C at 2.5 bar. Continental's page writes the rule as "every 12 degrees Celsius (approx. 10 degrees Fahrenheit)"; by our arithmetic, 1 psi corresponds to about 5.6 °C, so expect a little more change per degree Celsius than that wording suggests.
Set side by side, which is our reading: a tyre set at 36 psi in a 20 °C garage reads 3.5 psi low on a frosty morning, whatever is inside it. The whole year's advantage of nitrogen in the Consumer Reports test, 1.3 psi, is worth about 7.5 °C of cooling. Nitrogen does not stop the low-pressure warning coming on in the first cold snap; Transport Canada notes that large outside temperature changes can trigger it (Transport Canada); what a steady or a flashing warning means is explained in TPMS light flashing or solid. Our tyre pressure guide explains how to read pressures cold and what the warning light can and cannot tell you.
Water: the part that is real, and where it counts
The one physical difference that the gas law does not cover is water. Dry gas follows the table above. Liquid water inside a tyre does not: as it warms, more of it evaporates, and the vapour adds its own pressure. The USTMA says dry nitrogen is used in professional racing "to help reduce variation in inflation pressures (caused by moisture)".
How much can water add? This is our calculation from the NIST steam tables, for a tyre set at 2.5 bar at 20 °C with liquid water left inside:
| Tyre gas temperature | Dry gas reads | With liquid water present | Extra from water | Water needed to keep it saturated (40 L cavity) |
|---|---|---|---|---|
| 20 °C | 2.50 bar | 2.50 bar | 0 | 0.7 g |
| 60 °C | 2.98 bar | 3.15 bar | +0.17 bar (2.5 psi) | 5.2 g |
| 80 °C | 3.22 bar | 3.67 bar | +0.45 bar (6.5 psi) | 11.7 g |
The 40 L cavity is a rough upper bound for a 225/45 R18 tyre, taken as a rectangle from rim to tread. The extra pressure only appears if there is enough liquid water to evaporate: a few grams. At 20 °C, that cavity can hold only 0.7 g of water as vapour, so anything beyond that is liquid that came in with the air, for example from a compressor's tank or line. We have no measurement of how much water a typical forecourt compressor delivers, so we cannot tell you how often this happens on the road. What the table shows is why a racing team wants dry gas: the USTMA describes racing as a case where "even small differences in pressure can affect vehicle handling at the extreme limits of performance". The gain comes from "dry", not from "nitrogen". We found no source for the gas temperature inside a road tyre in normal driving, so we do not put a figure on it; the vehicle maker's pressures are set cold in any case.
Where nitrogen is required: aircraft
In US civil aviation, nitrogen is an airworthiness requirement for large aeroplanes. 14 CFR 25.733 (e) says that for an aeroplane with a maximum take-off weight of more than 75,000 lb (about 34 tonnes), "tires mounted on braked wheels must be inflated with dry nitrogen or other gases shown to be inert so that the gas mixture in the tire does not contain oxygen in excess of 5 percent by volume", unless the tyre liner will not produce a volatile gas when heated or the tyre temperature is otherwise kept safe (eCFR, 14 CFR 25.733; amendments of 1979, 1990 and 1993). The FAA's guidance on aircraft tyre maintenance tells technicians to make the initial inflation of tubeless tyres with dry nitrogen so that the tyre holds no more than 5 per cent oxygen, and to maintain loaded tyres with nitrogen (FAA AC 20-97B, 18 April 2005).
The reason is fire, not leaks. Heavy braking can heat the wheel and the tyre's inner liner until it gives off flammable gases; with little oxygen inside, they cannot burn. The USTMA gives the same reason for aircraft, commercial and heavy vehicles and mining equipment: to help reduce the risk of internal combustion if the brake, rim or wheel components overheat. Continental adds construction. None of the tyre makers we read extends this reason to passenger cars.
What the rules say for cars, country by country
None of the texts we read in the United Kingdom, the United States, Canada or Australia requires or forbids nitrogen in a car tyre. What they all require or advise is the right pressure, checked regularly:
| Country | Text | What it says about pressure checks |
|---|---|---|
| United Kingdom | The Highway Code, Annex 6 (updated 22 October 2025) | "Check weekly. Do this before your journey, when tyres are cold." Tyres "MUST be correctly inflated to the vehicle manufacturer's specification for the load being carried." |
| United States | FMVSS 138, statement required in the owner's manual of cars with a pressure monitor | "Each tire, including the spare (if provided), should be checked monthly when cold" and inflated to the vehicle maker's pressure. |
| Canada | Transport Canada, tire pressure monitors | Monitoring systems do not replace the maintenance in the owner's manual; keep measuring "on a monthly basis". |
| Australia | No national text read on this point; Continental's Australian page | Check at least once every two weeks and before a long journey. |
Sources: Highway Code, Annex 6; 49 CFR 571.138; Transport Canada; Continental, Australia. In the UK, the MOT inspects the pressure monitoring system of cars that have one, not the gas in the tyres; see the MOT and TPMS with aftermarket wheels.
Pros and cons on a road car
| Claim | What the sources support |
|---|---|
| Holds pressure longer | Yes, a little: 1.3 psi less loss in a year in Consumer Reports' test. Not against punctures, bead, valve or wheel leaks (USTMA, Michelin). |
| Better fuel economy | Only through pressure: at the right pressure, rolling resistance is the same with either gas (Consumer Reports, citing NHTSA; Continental). |
| Tyres last longer | Less oxidation inside, according to the NHTSA study as Consumer Reports reports it; not a practical concern for most drivers, who wear tyres out first. Continental sees no discernible benefit for ageing. |
| Steadier pressure when hot | Only the dry part counts, and it matters at racing temperatures (USTMA). |
| Safer | Only if it keeps a tyre nearer the right pressure; the USTMA warns that relying on nitrogen to check less often can lead to underinflation. |
| Cost and convenience | A fill costs money and is not available at every forecourt. Consumer Reports reports charges of $5 or more per tyre at dealerships and tyre shops. |
| Mixing with air | Harmless: "any and all proportions" (USTMA). It reduces the share of nitrogen. |
How much nitrogen is in a "nitrogen" tyre? Our calculation: deflating a tyre to atmospheric pressure and refilling it to 2.5 bar with 95 per cent nitrogen gives about 90 per cent nitrogen after one purge, 93.6 per cent after two and 94.6 per cent after three. A top-up of 0.3 bar of air into a 95 per cent tyre brings it back to about 93.5 per cent. This is why Consumer Reports asks the retailer to remove all the original air first and to keep topping up with nitrogen if you want the benefit. It also says there is no harm in adding air.
If your car came with green valve caps
Green caps usually mean the tyres were filled with nitrogen. Nothing changes in how you look after them. Check them cold at the interval in your handbook, inflate to the vehicle maker's pressure on the door-jamb label or in the handbook, and use air when nitrogen is not at hand: the USTMA says nitrogen-filled tyres "can and should have air added whenever nitrogen is not readily available". If a tyre keeps losing pressure faster than the others, look for a cause, not a gas: our guide to slow punctures goes through valves, beads and wheel damage. Before winter, the pressure check described in winter tyres in the UK applies whatever the gas, and wheels put away for the season are best stored as described in how to store tyres and wheels.
If a tyre is well below its pressure, or you find damage on the sidewall, have it checked before you drive on.
Frequently asked questions
Is nitrogen better than air in car tyres?
Slightly, for pressure retention: in Consumer Reports' year-long test, nitrogen tyres lost 2.2 psi against 3.5 psi for air. The USTMA, Michelin and Continental all say it is not necessary on a passenger car and that pressure checks are still required.
Can I mix nitrogen and air?
Yes. The USTMA says the two can be mixed "in any and all proportions" and that you should add air whenever nitrogen is not readily available. Michelin says the two "CAN be mixed, if needed". Mixing reduces the share of nitrogen, which is the only drawback.
Does nitrogen improve fuel economy?
Not directly. Consumer Reports quotes the NHTSA study: nitrogen had "little or no direct effect" on rolling resistance. Fuel economy depends on keeping the right pressure, and air does that as well if you check it.
Does nitrogen stop the tyre pressure warning light coming on in cold weather?
No. Dry nitrogen and dry air lose the same pressure when they cool: about 3.5 psi for a 20 °C drop at 36 psi, by our calculation. Transport Canada notes that large temperature changes can bring the warning on.
Do I still need to check tyres filled with nitrogen?
Yes. The UK Highway Code says weekly, the US owner's-manual statement and Transport Canada say monthly, Continental every two weeks. The USTMA says nitrogen alone "is not a replacement for regular inflation pressure maintenance".
Why do aircraft and racing cars use nitrogen?
Aircraft over 75,000 lb must, under 14 CFR 25.733 (e), keep oxygen in braked-wheel tyres at 5 per cent or less, to prevent fire if the tyre liner overheats. Racing teams use dry nitrogen to remove moisture, which makes hot pressures vary, according to the USTMA.
Does nitrogen make tyres last longer?
The NHTSA study, as Consumer Reports reports it, found that nitrogen reduces oxidation of the rubber, but Consumer Reports adds that most drivers replace tyres through wear long before that matters. Continental sees no discernible benefit for tyre ageing.
Is nitrogen in tyres legal or required anywhere for cars?
Not in any text we read for the United Kingdom, the United States, Canada or Australia. The legal requirement is the right pressure for the load, set by the vehicle maker.
Wheels and tyres together are a single fitment: Onyx Forged does not recommend a size or an offset, nor a tyre or a pressure, which come from your car's maker. Onyx Forged is a French brand of made-to-order forged wheels, designed to JWL/VIA standards: see the catalogue.
Sources
- USTMA, TISB Vol. 44 No. 3, Using nitrogen to inflate passenger and light truck tires (June 2024).
- Michelin US, tire pressure, section on nitrogen (updated 28 April 2026); Continental, nitrogen in tyres (Australian edition).
- Consumer Reports, should you use nitrogen in your car tires? (updated 11 September 2026), including the NHTSA findings it reports.
- eCFR, 14 CFR 25.733, tires; FAA, Advisory Circular 20-97B, aircraft tire maintenance (18 April 2005).
- GOV.UK, The Highway Code, Annex 6 (updated 22 October 2025); 49 CFR 571.138, FMVSS No. 138; Transport Canada, tire pressure monitors (modified 6 August 2021).
- NIST Chemistry WebBook, saturation properties of water.
All pages were read on 16 October 2026. The calculations in the temperature, water and purge sections are ours.
The Onyx Forged design office.