Fuel Economy Conversions

A car rated at 35 miles per gallon in the US would be listed as 42 miles per gallon in the UK, despite identical fuel consumption. The difference is the gallon: the US gallon is 3.785 liters while the imperial gallon is 4.546 liters. Since the car travels the same distance on the same amount of fuel, dividing by a larger gallon produces a bigger number. This is not a minor discrepancy. When a European buyer reads a US car review quoting 28 mpg and assumes imperial gallons, they expect roughly 10 liters per 100 km. The actual figure is closer to 8.4 L/100km. That misunderstanding could influence a purchasing decision worth tens of thousands of dollars.

Most of Europe, along with Australia, Canada (on paper), and much of Asia, uses liters per 100 kilometers. This unit measures fuel consumption rather than fuel economy, and its inverse relationship with MPG creates a non-linear conversion. Going from 10 L/100km to 8 L/100km saves 2 liters over 100 km. Going from 6 L/100km to 4 L/100km also saves 2 liters over 100 km. But in MPG terms, the first improvement is from 23.5 to 29.4 mpg (a 25% improvement), while the second is from 39.2 to 58.8 mpg (a 50% improvement). The same absolute fuel saving looks far more dramatic in MPG. This is why researchers like Duke University economist Richard Larrick have argued that L/100km is a better unit for consumers: it makes fuel savings linearly comparable.

The inverse conversion formula is: L/100km = 235.215 / MPG(US), derived from the exact gallon-to-liter relationship. For imperial MPG: L/100km = 282.481 / MPG(UK). The km/L unit, common in parts of Asia and Latin America, sits between the two conceptually: higher numbers mean better efficiency (like MPG) but the metric is fully metric. Converting between km/L and L/100km is a simple reciprocal: km/L = 100 / (L/100km).

Electric vehicles have added a new dimension. The US EPA rates EVs in MPGe (miles per gallon equivalent), where 33.7 kWh of electricity is defined as equivalent to one gallon of gasoline. A Tesla Model 3, rated at roughly 132 MPGe, consumes about 25 kWh per 100 miles, or 15.5 kWh/100km in European terms.

Real-world fuel economy also depends on driving conditions. The US EPA rates vehicles using a combination of city and highway cycles, yielding a "combined" MPG. European WLTP (Worldwide Harmonised Light Vehicles Test Procedure) testing produces L/100km figures that are generally closer to real-world driving than the older NEDC cycle. A car rated at 6.5 L/100km on the WLTP cycle might achieve 36.2 mpg (US) or 43.5 mpg (UK). These cross-standard comparisons come up frequently in car reviews, import decisions, and fleet management for companies operating vehicles across multiple countries.

This converter supports 4 fuel economy units: kilometers per liter (base unit), liters per 100km, miles per US gallon, and miles per imperial gallon. The L/100km conversion uses a reciprocal (100/value), not a linear factor, so the converter handles this non-linear relationship correctly. All factors derive from the exact gallon definitions per NIST Handbook 44.