Fuel Efficiency Converter

Convert fuel efficiency free online between MPG, L/100km, and km/L. These units measure efficiency in inverted ways, so the conversion isn't a simple multiplication.

Value

From

Browser only -- uses US gallon/mile definitions.

How to Use This Tool

Enter a fuel efficiency valueFrom a vehicle spec or fuel economy label

Choose from and to unitsMPG, L/100km, or km/L

See the converted resultCorrectly handles the inverse relationship

Higher-Is-Better Versus Lower-Is-Better -- The Real Complication

Most unit conversions scale linearly, but fuel efficiency units measure the relationship in genuinely opposite directions. MPG and km/L both increase with better efficiency; L/100km decreases with better efficiency. Converting between a "higher is better" and "lower is better" unit requires inverting the relationship mathematically, not just applying a scaling factor.

Why the Same MPG Improvement Means Different Real Savings

Going from 10 to 20 MPG saves far more fuel over a fixed distance than going from 40 to 50 MPG, even though both represent the same 10 MPG numeric increase. This countereasy-to-use fact is a direct mathematical consequence of the inverse relationship, and it has real practical implications for understanding where fuel efficiency improvements matter most.

Regional Convention Reflects Broader Measurement Habits

The US uses MPG consistent with its general imperial measurement convention, while most metric countries use L/100km. Neither measures something fundamentally different -- they're just expressing the same underlying efficiency concept in directions that fit each region's broader unit system.

Frequently Asked Questions

MPG measures distance per unit of fuel (higher is better), while L/100km measures fuel consumed per fixed distance (lower is better) -- these are inverse relationships, not just different scaling of the same measurement, which is why the conversion formula involves division, not a direct multiplier.

L/100km = 235.215 / MPG (using US gallons and miles). The 235.215 constant comes from the unit conversions between gallons/miles and liters/kilometers, combined with inverting the measurement direction.

This follows each region's broader measurement system convention -- metric countries naturally express fuel consumption in liters per 100 kilometers, while the US, using imperial units generally, expresses it as miles per gallon.

No, km/L (kilometers per liter) is a 'higher is better' measurement like MPG, while L/100km is a 'lower is better' measurement -- they're both metric-friendly but represent the efficiency relationship in opposite directions.

Because of the inverse relationship, improving from 10 MPG to 20 MPG saves dramatically more fuel over a fixed distance than improving from 40 MPG to 50 MPG, even though both represent a 10 MPG increase -- a countereasy-to-use but mathematically real consequence of the inverse relationship that matters for understanding real fuel savings.