Fuel Consumption

Fuel consumption describes how much fuel a vehicle uses to travel a given distance, and it is the single most important measure of how economical a car is to run. For most owners it translates directly into the cost of motoring, and for society it links to carbon emissions and energy use. Because it captures the efficiency of the whole vehicle — engine, transmission, aerodynamics, weight and rolling resistance combined — it is a far more meaningful real-world figure than any single mechanical specification.

The measure is expressed in two principal ways depending on the market. Much of the world quotes litres per 100 kilometres (L/100 km), where a lower number is better because it represents less fuel burnt over a fixed distance. Britain and the United States traditionally use miles per gallon (mpg), where a higher number is better, though the imperial gallon used in the UK is larger than the US gallon, so the two mpg figures are not directly interchangeable. Both conventions describe the same underlying relationship between fuel used and distance covered.

Fuel consumption is determined by a combination of fixed vehicle characteristics and variable driving conditions. A heavier car, a larger or less efficient engine, poor aerodynamics, under-inflated tyres and a high state of tune all increase consumption. So too does the way a car is driven: hard acceleration, high motorway speeds, frequent stops, cold short journeys, heavy loads and the use of air conditioning all push figures up. Steady, moderate driving in warm conditions yields the best returns.

Official consumption figures are obtained under standardised laboratory test procedures so that different models can be compared on an equal footing. The current European procedure is the Worldwide Harmonised Light Vehicles Test Procedure (WLTP), which replaced the older and notoriously optimistic NEDC cycle. WLTP figures are more realistic, but a gap inevitably remains: real-world consumption is almost always worse than the certified figure because everyday driving rarely matches the controlled test conditions. Owners should treat official numbers as a comparative benchmark rather than a promise.

Crucially, fuel consumption moves in lockstep with carbon dioxide emissions. Burning a litre of petrol or diesel produces a fixed quantity of CO2 — roughly 2.3 kg for petrol and 2.6 kg for diesel — so the amount of fuel consumed directly dictates the amount of CO2 emitted. This is why economy and environmental impact are inseparable, and why CO2-based taxation effectively penalises thirsty cars.

Fuel consumption sits at the centre of a cluster of related concepts. It is expressed through the units litres per 100 km and miles per gallon, is tied physically to CO2 emissions, and is officially certified under the WLTP procedure that governs how the headline figures on a car's specification sheet are derived.