Fuel Consumption Converter

Fuel consumption is measured differently depending on where you are in the world. In the United States and the United Kingdom, fuel economy is expressed as miles per gallon — how far a vehicle travels on a single gallon of fuel. In most of Europe, Australia, and Asia, the standard is litres per 100 kilometres — how many litres of fuel a vehicle consumes to travel 100 km. Canada often uses both. Understanding these units and how to convert between them is essential for comparing vehicles across markets, calculating running costs, and planning long road trips.

MPG vs L/100km: Inverse Relationships

The key insight when working with fuel efficiency units is that MPG and L/100km are inversely proportional — a higher MPG means better efficiency, while a lower L/100km also means better efficiency. This inverse relationship means you cannot simply apply a linear multiplier to convert between them. A car that achieves 30 mpg (US) consumes approximately 7.8 L/100km. A car that does 50 mpg (US) consumes about 4.7 L/100km. The formula is: L/100km = 235.21 ÷ mpg (US). This non-linear relationship means that small improvements in MPG at low efficiency levels represent proportionally larger fuel savings than the same improvement at high efficiency levels.

US and UK gallons are not the same size, which is why MPG figures differ between the two countries even for identical vehicles. A US gallon is 3.785 litres, while a UK (imperial) gallon is 4.546 litres — about 20% larger. A car rated at 40 mpg UK would achieve approximately 33 mpg US for the same journey. When comparing fuel economy figures between American and British sources, always check which gallon standard is being used to avoid a significant misunderstanding.

What Affects Real-World Fuel Consumption?

Official fuel economy ratings are measured under controlled laboratory conditions and often do not reflect real-world driving. Speed is one of the biggest factors: aerodynamic drag increases with the square of velocity, so driving at 110 km/h uses significantly more fuel per kilometre than driving at 80 km/h. Aggressive acceleration and hard braking also consume substantially more fuel than smooth, anticipatory driving. Air conditioning can reduce fuel economy by 5–25% depending on the vehicle and conditions, while cold engines run richer fuel mixtures until they reach operating temperature, reducing economy on short trips.

Tyre pressure, vehicle load, roof racks, and even fuel grade all play measurable roles in real-world consumption. Modern cars with onboard computers typically display instantaneous and average consumption figures, but these can drift from actual figures due to calibration. The most accurate way to measure real-world fuel economy is the fill-to-fill method: fill the tank completely, record the odometer, drive normally, fill again, and divide the distance driven by the fuel added.

Calculating Fuel Costs for a Journey

Once you know a vehicle's fuel consumption rate and the local fuel price, calculating the cost of any journey is straightforward. Multiply the journey distance by the consumption rate to get the fuel volume required, then multiply by the price per litre or gallon. For example, a 500 km trip in a car that consumes 8 L/100km requires 40 litres of fuel. At $1.80 per litre, the fuel cost is $72. The same calculation in imperial terms: a 300-mile trip in a car achieving 35 mpg (US) requires approximately 8.6 gallons. At $3.50 per gallon, the cost is around $30.

For fleet operators, long-distance commuters, and anyone comparing the operating costs of two vehicles, these calculations make a meaningful difference. A vehicle that achieves 7 L/100km instead of 10 L/100km saves 3 litres per 100 km — at scale, this represents thousands of dollars in annual savings. The fuel cost calculator embedded in our converter makes it easy to compare two vehicles side by side on any route.

Electric Vehicles and Energy Efficiency

As electric vehicles become mainstream, energy efficiency is increasingly expressed in miles per kilowatt-hour (mi/kWh) or its inverse, kilowatt-hours per 100 kilometres (kWh/100km). These units serve the same purpose as MPG and L/100km but for electricity rather than liquid fuel. Converting between electric efficiency and equivalent fuel economy is done through the concept of MPGe (miles per gallon equivalent), where 33.7 kWh is defined as the energy content of one gallon of petrol. A car achieving 3 mi/kWh has an MPGe of approximately 100 — more than twice the efficiency of a typical 40 mpg petrol car. Understanding these conversions helps consumers compare the true running costs of electric and combustion vehicles on an equal footing.