L/100 km

Litres per 100 kilometres is the standard metric measure of a vehicle's fuel consumption, expressing how many litres of fuel are needed to travel a fixed distance of one hundred kilometres. It is the figure quoted on official documents, sales material and trip computers across continental Europe and most of the world, and it sits at the heart of how buyers compare the running costs of one car against another. Its logic is that of consumption: it answers the question of how much fuel a journey will swallow.

The unit is, in effect, an inverse of the older fuel-economy measures. Because it counts fuel used for a set distance rather than distance covered per unit of fuel, a lower number is better — a car rated at five litres per 100 km is more economical than one rated at eight. This runs directly counter to the miles-per-gallon convention familiar in Britain and North America, where a higher number signals greater efficiency, and the two systems therefore move in opposite directions. Conversion between them is not a simple linear scaling, since one expresses distance over fuel and the other fuel over distance, so they must be related through their reciprocals.

One of the practical virtues of the litres-per-100-km figure is that it scales in a straightforward, additive way. Because consumption is expressed against distance, the fuel needed for any trip is found simply by multiplying the rating by the number of hundred-kilometre units covered, and the consumption of mixed driving can be averaged sensibly. This makes the measure easy to translate into real money: multiplied by the price of fuel, it gives a clear cost per hundred kilometres, which is why it is so useful for budgeting and for weighing the lifetime expense of one model against another.

The figure also bears a close relationship to a car's carbon dioxide output. For a given fuel, every litre burned releases a roughly fixed mass of carbon dioxide, so consumption in litres per 100 km translates almost directly into grams of carbon dioxide per kilometre, the metric on which much vehicle taxation and emissions regulation is based. Petrol and diesel differ slightly in their energy content and carbon content per litre, which is why the two fuels are not directly comparable litre for litre, but within each fuel the link between consumption and emissions is tight.

Quoted consumption figures are produced under standardised laboratory test cycles so that different cars can be compared on an equal footing. The current European standard is the Worldwide Harmonised Light Vehicle Test Procedure, or WLTP, introduced to give figures closer to real-world use than the older and notoriously optimistic NEDC cycle it replaced. Even so, official figures remain idealised, and actual consumption varies with driving style, load, terrain, weather and traffic. Read with that caveat in mind, litres per 100 km remains the clearest single yardstick of how thirsty a car is and what it will cost to run.