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Fuel economy vs consumption

By RoadCost Editorial4 min readUpdated

People often use fuel economy and fuel consumption as if they mean the same thing. They are related, but they point in opposite directions. That matters when you compare cars, estimate savings or explain a result to someone else.

Quick answer

Fuel economy is distance per fuel: MPG or km/L, where higher is better. Fuel consumption is fuel per distance: L/100 km, where lower is better.

In this guide

  1. 1. A simple way to remember it
  2. 2. Why L/100 km is often clearer
  3. 3. Where each unit fits
  4. Questions

Key takeaways

  • MPG and km/L are economy units.
  • L/100 km is a consumption unit.
  • Do not compare the two directions without converting.

A simple way to remember it

Economy answers: how far can I go on a fixed amount of fuel? Consumption answers: how much fuel do I need for a fixed distance? Both are useful, but they behave differently when you change the numbers.

If a car improves from 30 to 40 MPG, that sounds like a big gain. In L/100 km terms, the change may look smaller or larger depending on where you started. Consumption is linear, which makes improvements easier to compare.

Why L/100 km is often clearer

A drop from 8 to 7 L/100 km saves one litre every 100 km. The relationship is direct. MPG is an inverse measure, so a 5 MPG improvement near the top of the scale saves less fuel than the same 5 MPG improvement at the bottom.

For cost work in metric countries, L/100 km is usually the fastest route: litres = kilometres × L/100 km ÷ 100.

litres used = km × (L/100 km) ÷ 100
cost = litres used × price per litre

Where each unit fits

Use MPG or km/L when that is the local convention and your audience expects it. Use L/100 km when you need a linear comparison, a fuel-cost calculation, or a clear statement that lower is better.

The important thing is to label the unit. An unlabelled number is not a useful fuel-economy claim.

Questions

Is MPG fuel economy or consumption?
MPG is fuel economy: miles per gallon. Higher is better.
Is L/100 km better than MPG?
Neither is universally better. L/100 km is linear and easier for cost comparisons; MPG is the familiar convention in some markets.
Why does MPG go down when L/100 km goes up?
They measure opposite things. More fuel per distance means fewer miles per gallon.

Sources

Try the calculator

Fuel efficiency converter

See economy and consumption side by side.

Related guides

MPG to L/100 km: exact conversion

Convert MPG to L/100 km with the right constant for UK and US gallons, including worked examples and common mistakes.

L/100 km to MPG: exact conversion

Turn litres per 100 kilometres into UK or US MPG with the exact formulas, examples and a simple way to remember the ratio.

km/L to MPG: Japan, India and Korea

Understand kilometres per litre, convert it to MPG UK or US, and see why km/L is the inverse of L/100 km.

A note on prices and rules

This guide explains arithmetic and units. It does not publish live fuel prices, electricity tariffs or changing tax rules. Use your own price and check official sources for anything that changes.

Page last reviewed: . Formulas are covered by automated tests. Prices are inputs, not live data.