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Automotive guide

Fuel Efficiency Units Explained

Fuel efficiency can be shown as distance per fuel unit, such as MPG, or fuel used per distance, such as L/100 km. These units move in opposite directions.

MPG vs L/100 km

Higher MPG means better efficiency because the vehicle travels more miles per gallon. Lower L/100 km means better efficiency because the vehicle uses fewer liters to travel 100 kilometers. This inverse relationship is why fuel-efficiency conversions need a proper formula.

Fuel cost estimates

A fuel cost calculator combines distance, efficiency and fuel price. The core idea is: fuel used x price per fuel unit = trip cost. The result is an estimate because real driving conditions, traffic, cargo, weather and driving style can change consumption.

What to keep consistent

  • Distance units: do not mix miles and kilometers without conversion.
  • Fuel units: gallons can mean US gallons or imperial gallons.
  • Trip direction: mark round trips clearly instead of doubling the distance mentally.
  • Currency: currency affects display only; it does not change fuel use.

Common mistakes

  • Assuming 30 MPG and 30 L/100 km mean similar efficiency. They do not.
  • Using imperial gallons in a US MPG formula.
  • Comparing fuel prices without checking whether the price is per liter or per gallon.
  • Forgetting that electric vehicles use energy units such as kWh, not liquid fuel units.

MPG and L/100 km conversion examples

US MPG and L/100 km move in opposite directions: higher MPG is better, lower L/100 km is better. The common US conversion is L/100 km = 235.214583 / MPG. A car rated at 30 US MPG is therefore about 7.84 L/100 km. A car using 6.0 L/100 km is about 39.2 US MPG.

Imperial MPG is different because an imperial gallon is larger than a US gallon. Fuel calculators need to state which gallon is used and must not mix miles, kilometers, liters and gallons without clear labels.

Useful calculators

FAQ

Is higher MPG always better?

For the same fuel type and measurement system, yes. Higher MPG means more distance per unit of fuel.

Is lower L/100 km better?

Yes. Lower L/100 km means less fuel used for the same distance.

Why does my real fuel cost differ from the estimate?

The estimate uses the efficiency and price you enter. Real-world consumption changes with speed, traffic, load, terrain and weather.

References

Last reviewed: 2026-05-16.