About the Fuel Economy Converter
Fuel economy is measured in miles per gallon in the US and liters per 100 kilometers in most of the rest of the world - and confusingly, these two units move in opposite directions (higher mpg is better, lower L/100km is better). Our Fuel Economy Converter translates between them.
How It Works
The calculator applies the standard conversion constant (235.215) that relates the two measurement systems, since mpg measures distance per unit of fuel while L/100km measures fuel per unit of distance - inverse relationships that this conversion handles correctly.
Formula & Methodology
Miles per gallon expresses distance traveled per fixed unit of fuel, while liters per 100 kilometers expresses fuel consumed per fixed unit of distance - they're structured as inverses of each other, which is exactly why converting between them requires division rather than a simple multiplication by a constant ratio the way most unit conversions work. The constant 235.215 itself comes from combining the mile-to-kilometer distance ratio with the US-gallon-to-liter volume ratio into a single figure that makes the inverse conversion work in one step.
Step-by-Step: Calculating It By Hand
- 1To convert mpg to L/100km: divide 235.215 by the mpg value.
- 2To convert L/100km to mpg: divide 235.215 by the L/100km value.
- 3Double-check which direction you need - remember higher mpg is better, while lower L/100km is better.
- 4For UK imperial mpg (a different gallon size than US mpg), use a separate conversion constant rather than this US-based one.
Examples
US to metric
30 mpg converts to approximately 7.84 L/100km - useful when comparing a US vehicle's fuel economy rating against international specifications.
Understanding the inverse relationship
A lower L/100km number means better fuel economy (using less fuel per distance), while a higher mpg number means better fuel economy - the two scales move in opposite directions, which is easy to misread at a glance.
Advantages
- Correctly handles the inverse relationship between mpg and L/100km
- Uses the precise standard conversion constant for accurate results
- Useful for comparing vehicle specifications across US and international markets
- Fast conversion in either direction
Common Mistakes
- Forgetting mpg and L/100km move in opposite directions - a common source of confusion when comparing vehicles
- Assuming US mpg and UK/Imperial mpg are the same, when they're based on different gallon sizes and produce different results
- Not double-checking which direction a specific comparison requires before drawing a conclusion
- Comparing fuel economy ratings across different testing standards (EPA vs. WLTP, for example) without accounting for methodology differences
Edge Cases to Watch For
- UK imperial gallons are about 20% larger than US gallons, so 'UK mpg' and 'US mpg' figures for the identical vehicle are meaningfully different numbers and shouldn't be compared directly.
- Because the relationship is inverse, a small mpg value change near the low end of the scale produces a much larger L/100km change than the same mpg change near the high end.
- Fuel economy testing standards differ by region (EPA in the US, WLTP in Europe, and others), so even a correctly unit-converted figure may not be directly comparable if the underlying test methodology differs.
- Electric vehicle efficiency uses entirely different units (like miles per kWh or kWh per 100 km) that this converter, built for combustion-engine fuel economy, doesn't cover.
Common Use Cases
- Comparing vehicle fuel economy specifications between US and international markets
- Understanding a car's fuel efficiency when shopping internationally
- General automotive research and vehicle comparison
- Converting a specific known fuel economy figure between unit systems