Fuel Economy Converter

Fuel economy conversion is the process of expressing a vehicle's fuel efficiency in a different unit of measurement. Different regions of the world use different standards: the United States measures fuel economy in miles per gallon (US MPG), while Europe, Australia, and most of the world use liters per 100 kilometres (L/100km). Japan, much of Asia, and Latin America often use kilometers per liter (km/L). The United Kingdom officially uses L/100km but historically quoted miles per gallon (UK MPG) using the larger imperial gallon.

One key nuance of fuel economy conversion is the inverse relationship between MPG and L/100km. Unlike most unit conversions, a higher MPG number means better fuel efficiency, while a lower L/100km number also means better efficiency. This means that doubling your MPG does not halve your L/100km value—the relationship is non-linear and inverse (L/100km = constant ÷ MPG). Understanding this is crucial when comparing international car specifications or calculating fuel costs for trips abroad.

According to the US Department of Energy's fueleconomy.gov, knowing how to interpret fuel economy figures from different countries helps consumers make better-informed purchasing decisions and accurately estimate running costs when driving internationally.

Getting the most out of fuel economy figures requires understanding a few key concepts. Here are five tips to help you interpret and convert fuel efficiency data accurately:

1. Understand the Inverse Relationship

MPG and L/100km are inversely related. A higher MPG means better efficiency; a lower L/100km also means better efficiency. A 10% improvement in L/100km is not the same fuel saving as a 10% improvement in MPG. When comparing two cars, convert both values to the same unit before drawing conclusions.

2. Know the US vs UK MPG Difference

Always check which gallon a fuel economy figure uses. US MPG is based on the US liquid gallon (3.785 litres), while UK MPG uses the imperial gallon (4.546 litres). A car quoted at 50 mpg in the UK is only about 41.6 mpg US — a significant difference that catches many buyers off guard when comparing international specifications.

3. Use L/100km for Cost Calculations

L/100km is the most convenient unit for calculating fuel costs. The formula is straightforward: fuel cost = (L/100km × distance km ÷ 100) × price per litre. Because L/100km scales linearly with distance and fuel price, it is easier to use than MPG for budgeting road trips and comparing running costs.

4. Distinguish City vs Highway Figures

Most manufacturers publish separate city and highway (motorway) fuel economy figures. Highway driving typically produces 20–40% better fuel economy than city driving due to fewer stops and lower engine load. When converting for comparison, ensure you are comparing like-for-like — both city, both highway, or both combined figures.

5. Electric Vehicle Equivalent (MPGe)

For electric vehicles, the US EPA uses MPGe (miles per gallon equivalent), which converts electrical energy to a petrol equivalent. 1 US gallon of petrol contains approximately 33.7 kWh of energy. MPGe figures are typically much higher than conventional MPG — for example, 100 MPGe ≈ 2.35 L/100km equivalent — but they represent electricity consumption, not liquid fuel, so direct comparisons require additional context.