Fuel Economy Converter
Fuel economy measures vehicle efficiency as distance travelled per unit of fuel consumed, but different regions use different units, creating confusion when comparing vehicles internationally. The United States uses miles per gallon (mpg), calculated as distance divided by fuel volume. The imperial gallon used in the United Kingdom and some Commonwealth countries is larger than the US gallon, making UK mpg higher than US mpg for the same vehicle. Most countries use kilometres per litre (km/L), where higher is better. Europe frequently uses litres per 100 kilometres (L/100km), an inverse measurement where lower is better. These units do not convert linearly because L/100km is volume per distance, not distance per volume. A vehicle that achieves 30 mpg (US) travels 30 miles on one US gallon, equivalent to 12.7 km/L or 18.6 L/100km. Understanding fuel economy conversions is essential when buying or comparing vehicles from different countries, evaluating environmental impact, or calculating fuel costs. This calculator converts between the four most common fuel economy units.
NIST conversion factors
Fuel economy units do not all convert linearly. Distance-per-volume (mpg, km/L) use linear factors. Volume-per-distance (L/100km) uses the inverse formula: L/100km = 235.215 / mpg (US).
| Unit | Symbol | Conversion |
|---|---|---|
| Miles per gallon (US) | mpg (US) | Base unit (1) |
| Miles per gallon (Imperial) | mpg (Imperial) | 1.20095 |
| Kilometres per litre | km/L | 0.42514 |
| Litres per 100 kilometres | L/100km | 235.215 / mpg (US) |
Fuel economy converter: frequently asked questions
What is fuel economy?
Fuel economy measures how far a vehicle travels per unit of fuel consumed. The US uses miles per gallon (mpg), most of the world uses kilometres per litre (km/L), and Europe often uses litres per 100 km (L/100km). Higher mpg or km/L means better efficiency; lower L/100km means better efficiency.
Why does the US use miles per gallon?
Historical convention. The US customary system uses miles and gallons. Most countries adopted the SI system with kilometres and litres. This creates confusion when comparing vehicle efficiency internationally.
What is the relationship between these units?
Fuel economy uses distance-per-volume (mpg, km/L) and volume-per-distance (L/100km). The formula L/100km = 235.215 / mpg (US). This means you cannot use simple linear conversions for L/100km.
How precise are these conversions?
These use NIST-defined factors. 1 US gallon equals exactly 3.785411784 litres. The conversion factor 235.215 for L/100km is mathematically derived.
Why do I need this converter?
When comparing vehicles from different countries, understanding equivalent fuel economy is essential. A 30 mpg US vehicle and a 12.7 L/100km vehicle are equivalent. Comparisons help evaluate efficiency across borders.
Official sources
- NIST Handbook 44: Specifications, Tolerances, and Other Technical Requirements for Weighing and Measuring Devices.
- NIST Special Publication 330: The International System of Units.
Reviewed by the CalculatorHub team, edited by James Graham, 14 June 2026. See our methodology.