Fuel Economy: MPG vs L/100km Explained

Two Approaches to Measuring Fuel Efficiency

The United States measures fuel economy in miles per gallon (MPG), asking how far you can drive on a fixed amount of fuel. Most other countries use liters per 100 kilometers (L/100km), asking how much fuel you need for a fixed distance. These approaches are inversely related: higher MPG means better efficiency, while lower L/100km means better efficiency.

The conversion formulas are: L/100km = 235.215 / MPG and MPG = 235.215 / L/100km. This inverse relationship means the conversion is not a simple multiplication but a division.

Conversion Reference

Common fuel economy equivalents:

• 15 MPG = 15.7 L/100km (large SUV or truck)
• 20 MPG = 11.8 L/100km (midsize SUV)
• 25 MPG = 9.4 L/100km (typical sedan)
• 30 MPG = 7.8 L/100km (efficient sedan)
• 35 MPG = 6.7 L/100km (compact car)
• 40 MPG = 5.9 L/100km (hybrid)
• 50 MPG = 4.7 L/100km (efficient hybrid)
• 60 MPG = 3.9 L/100km (plug-in hybrid)

Note that US gallons and imperial gallons differ. If you see UK MPG figures, they will be higher than US MPG for the same vehicle because imperial gallons are larger. To convert UK MPG to US MPG, multiply by 0.832.

Why the Inverse Relationship Matters

The MPG scale is counterintuitively nonlinear in terms of fuel savings. Improving from 10 MPG to 15 MPG saves more fuel over a given distance than improving from 30 MPG to 50 MPG. This is because MPG improvements at the low end represent larger reductions in fuel consumption.

For example, driving 10,000 miles at 10 MPG uses 1,000 gallons. At 15 MPG, it uses 667 gallons, a savings of 333 gallons. Improving from 30 to 50 MPG changes consumption from 333 to 200 gallons, saving only 133 gallons. The L/100km scale makes this clearer because it is linear: each unit decrease represents the same fuel savings.

This is known as the MPG illusion, and it has real policy implications. Replacing a fleet of 15 MPG trucks with 20 MPG trucks saves more fuel than replacing 30 MPG sedans with 40 MPG hybrids.

Factors That Affect Fuel Economy

Fuel economy varies based on driving conditions, vehicle maintenance, and driver behavior. City driving typically produces lower MPG than highway driving due to frequent stops and acceleration. Tire pressure, engine tuning, air filter condition, and vehicle weight all impact consumption.

Driving habits matter significantly. Aggressive acceleration, high-speed cruising, and excessive idling all increase fuel use. Smooth acceleration, maintaining steady speeds, and anticipating traffic flow can improve economy by 10-30% compared to aggressive driving.

Weather and terrain also play roles. Cold starts in winter reduce efficiency, headwinds increase fuel consumption at highway speeds, and hilly terrain demands more energy than flat roads.

Electric and Hybrid Metrics

Electric vehicles use MPGe (miles per gallon equivalent) in the US, based on the energy content of a gallon of gasoline (33.7 kWh). A car rated at 120 MPGe is highly efficient. In metric countries, electric vehicle efficiency is measured in kWh per 100 km, providing a direct energy consumption figure.

Plug-in hybrids complicate things further because they use both electricity and gasoline. Their advertised fuel economy depends heavily on how much you drive in electric-only mode versus hybrid mode.

Use the unit conversion tools on CalcHub to convert between MPG and L/100km, or explore our math calculators for fuel cost analysis.

Compare fuel economy across systems with CalcHub’s converter.