CNG versus LPG: two gases, wrongly treated as one

You pull up at a filling station whose map pin lists both LPG and CNG and assume one nozzle covers both tanks. It does not. LPG is propane and butane, kept liquid under mild pressure. CNG is methane, squeezed to around 200 bar and stored as a gas. They get lumped together as gas cars, and that habit costs people a wasted stop at least once a year.

By the Refuelia Editorial Team · Reviewed 28 July 2026

Two gases, one lazy word

LPG stands for liquefied petroleum gas, and it is exactly what the name says: a mix of propane and butane, blended to a national specification, kept liquid by fairly modest pressure. The blend itself varies by country. The Advanced Motor Fuels programme, run under the International Energy Agency, notes that LPG sold in the Netherlands runs close to 60 percent propane and 40 percent butane, while colder markets such as Sweden, Canada and the United States lean much more heavily towards propane, because propane stays liquid at lower outdoor temperatures. That liquid state is maintained by a vapour pressure of roughly 210 to 810 kilopascals at 20 degrees Celsius, rising to at most 1550 kilopascals at 40 degrees, under the EN 589:2022 fuel quality standard. In round numbers, that is somewhere between 2 and 15 bar.

CNG stands for compressed natural gas, and the gas in question is methane. Methane will not stay liquid anywhere near ambient temperature and pressure. LNG gets around that by chilling it to minus 162 degrees, but a CNG car does something cruder: it just squashes the gas hard, typically to around 200 bar, with some systems built and rated to 250 bar. Both fuels get called gas in everyday speech, both get called gaz in French, and the confusion is baked into the vocabulary before a driver even reaches the pump.

Pressure, tanks and kerb weight

Because LPG only needs single or low double digit bar, its tank is a comparatively ordinary steel or composite cylinder, not far removed in engineering terms from a household gas bottle. CNG's 200 to 250 bar is a different problem altogether. The cylinder has to be built and certified to hold roughly ten to twenty times the pressure of an LPG tank, with thicker walls or wound composite construction, tested and stamped to match. A CNG cylinder of the same physical size as an LPG tank ends up heavier, and, as the next section shows, still holds less usable range.

Energy content and what it does to your range

LPG's lower heating value sits at 44 to 46 megajoules per kilogram, or 23 to 26 megajoules per litre, according to the same IEA fuel properties data. CNG carries more energy per kilogram, 53.6 megajoules according to standard engineering references, but because it stays a gas rather than becoming a liquid, its volumetric energy density at 200 to 250 bar works out to only around 9 megajoules per litre. That gap matters more than the per kilogram figure, because a fuel tank is a fixed volume, not a fixed weight. Litre for litre, LPG carries roughly two and a half times more usable energy than CNG at typical storage pressure. That is the real reason CNG cars tend to run shorter gas-only ranges than an equivalent LPG conversion with a similarly sized tank, and why factory CNG models usually carry a noticeably bigger, heavier cylinder just to close some of that gap.

The filler coupling problem

LPG's biggest practical headache is not the fuel, it is the nozzle. Europe never settled on one filling connector, and four incompatible types are still in daily use, according to a country by country guide compiled for LPG-using campervan owners:

• ACME, a threaded fitting used in Austria, Belgium, Germany, Ireland, Liechtenstein, Luxembourg, Switzerland and the UK
• Bayonet, a push and twist fitting used in Denmark, Norway, Spain, the Netherlands and the UK
• Dish, a concave fitting and the most widespread type, covering Austria, France, Italy, Poland and much of Eastern Europe
• Euronozzle, a standardised connector intended to eventually replace the others, currently limited mainly to Spain

Some countries, Austria and Spain among them, run more than one type at different stations. A driver taking an LPG car from Germany into Spain needs an adapter, and needs to know which one, before they leave. CNG filling connectors have a less tangled history: most European stations use a single recognised coupling design, so crossing a border is less of a lottery, even though the stations themselves are much harder to find in the first place.

Conversion cost and payback

LPG conversion in the UK averages around £1,200 for a petrol car, according to trade pricing guide Checkatrade, with savings of up to 50 percent against petrol and LPG itself priced well under £1 a litre when that pricing was compiled. For a driver covering typical annual mileage, that outlay tends to come back within a year or two of regular driving, faster for higher mileage, slower for occasional use.

CNG conversion is a bigger commitment. German motoring cost guide Bussgeldkatalog puts a retrofit at between 3,500 and 4,500 euros, plus roughly 100 euros for the mandatory gas system inspection, and is blunt that it only pays off for genuinely high mileage drivers. Weekend-only use rarely clears that bar. The higher cost reflects the tank itself: a 200 bar pressure vessel, certified and tested, is simply a more expensive thing to build and fit than an LPG cylinder rated to a few bar.

Where the network holds up, and where it thins out

LPG's network is large and mature. Community-maintained tracker myLPG.eu lists tens of thousands of stations worldwide, and within Europe Poland leads with around 6,283, Germany with around 5,214, Italy with around 4,879 and Turkey with around 4,582, based on its most recent update. CNG's network is a different order of magnitude smaller. CNG Europe's station tracker put the whole continent at a little over 4,100 stations as of February 2025, with Italy well ahead at 1,063 and Germany around 868, but coverage falls away fast outside those two markets: Great Britain had just 7 stations, and Luxembourg had none at all.

This is where the map itself becomes part of the problem, not just the fuel. LPG and CNG stations are far sparser than petrol pumps, so a wrong pin or a station that has quietly stopped selling gas is not a minor inconvenience, it is a real detour, sometimes a long one on a route with only two or three gas stops to begin with. Base map data can tell you which fuels a site is listed as selling. It cannot tell you whether the CNG compressor was actually working this morning, or whether the LPG pump has been out of service since last week. That gap is exactly why a timestamped driver report matters more for these two fuels than it does for petrol, where the next station is rarely more than a few minutes away regardless.

Safety, boot space and which driver each one suits

The two gases behave differently if they leak, and that difference drives real parking rules. Methane is lighter than air and disperses upward almost immediately. Propane and butane are heavier than air, sink towards the floor and, in an enclosed space like an underground car park, can pool into a mixture that is both explosive and easily ignited, as German LPG and CNG information site gibgas.de sets out plainly. Under Germany's state level garage regulations, CNG cars are now permitted in underground car parks without restriction, while LPG cars face conditions that vary by state, and individual operators can still refuse entry regardless of the rules. Anyone driving a gas car into an unfamiliar underground car park abroad should check local signage rather than assume home country rules travel with them.

Boot space follows from the same pressure difference already covered. A modern LPG retrofit typically uses a toroidal, ring-shaped tank that sits in the spare wheel well and leaves the boot floor untouched, at the cost of the spare wheel itself, usually replaced with a repair kit or a smaller emergency spare. A CNG tank, built to survive 200 bar rather than a handful, is a bulkier, heavier cylinder for the same stored energy, and on factory CNG cars it usually eats a genuine chunk of boot floor or sits under the load area in a way that shows up on the spec sheet as reduced litres, not just a missing spare wheel.

None of this makes one fuel better than the other, it makes them suited to different drivers. LPG suits someone doing ordinary mileage who wants a cheap conversion, a mature and dense station network across most of Europe, and does not want to lose boot space, but who also needs to carry the right nozzle adapter for wherever they are driving. CNG suits a high mileage driver, often a fleet or taxi operator, who can absorb a steeper conversion cost against years of cheaper fuel, is comfortable planning routes around a thinner station network, and does not mind giving up more boot space for a fuel that burns cleaner per kilogram, even if it is not the fuel to reach for on a spontaneous long drive with the tank already low.