MPG vs L/100km vs km/L: Global Fuel Efficiency Units Explained & Converted

Why Two Countries Use Two Different Units

MPG and L/100km are not just different numbers, they are answers to different questions. Miles per gallon tells you how far you can travel on a unit of fuel. Litres per hundred kilometres tells you how much fuel you need to travel a fixed distance. The two metrics are mathematically reciprocal (after unit conversion), but psychologically they push in opposite directions. A higher MPG is better; a lower L/100km is better. This is why the same car, advertised on opposite sides of the Atlantic, can carry both a 30 MPG sticker and a 7.8 L/100km sticker without anyone being misleading.

The historical reason for the split is regulatory. The United States standardised on miles and US gallons (3.785 L) through the Energy Policy and Conservation Act of 1975, which made MPG the de-facto efficiency metric for CAFE compliance. Europe, Japan, and most of the rest of the world adopted the metric system earlier and used L/100km because it pairs naturally with the litre (the standard fuel-trade unit) and the kilometre (the standard distance unit). The UK's transition to L/100km in primary fuel-economy reporting is more recent, and UK drivers still encounter MPG in older marketing material; the conversion is complicated by the fact that the UK traditionally used imperial gallons (4.546 L) rather than US gallons, so a "UK MPG" of 40 corresponds to a "US MPG" of approximately 33.

Quick Reference Conversion Table

For a representative range of modern passenger vehicles. All conversions use the US gallon (3.785 L) unless explicitly noted; for the UK gallon, multiply MPG (US) by 0.833 to get MPG (UK).

The Reciprocal Trap

A common mistake is to assume that halving L/100km doubles MPG. It does not. Because the two units are reciprocals, a 50% reduction in L/100km corresponds to a doubling of fuel efficiency per unit distance, which is the same as a doubling of MPG. But a 20% reduction in L/100km corresponds to a 25% increase in MPG, not a 20% increase. The asymmetry is small at high efficiencies (where the curve is approximately linear) and large at low efficiencies (where the curve bends). A car that goes from 5 MPG to 10 MPG has improved by 100% in MPG, but only by 50% in L/100km (from 47.0 to 23.5 L/100km). The two metrics tell the same story; they just bend differently.

Working With Mixed Units in Fleet Operations

If you operate a fleet that includes vehicles from both the metric and the imperial world, for example, a UK logistics company running a mix of European vans and American trucks, the safest practice is to pick one canonical unit for internal reporting and convert at the edges. Most large fleets standardise on L/100km because it is the unit the European OEMs use on the door-jamb labels, but if your drivers are American and your fuel cards report in US gallons, MPG (US) is the more practical choice. The important thing is to never mix the two within a single comparison; an "MPG (UK) of 33 versus an L/100km of 9.0" is meaningless because the two numbers describe different vehicles under different standards.