Conversions

Fuel Economy Converter

What this does

Convert fuel consumption between US mpg, UK mpg and liters per 100 km with exact triangular formulas; the mpg ↔ L/100km bridge.

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Calculator inputs

How to use it

Using the fuel economy converter

  1. 01

    Enter the rating

    Use the number from the window sticker or brochure.

  2. 02

    Say which gallon

    US and UK mpg differ by about 20%; pick the right source country.

  3. 03

    Compare all three

    The result shows both remaining units so nothing needs a second lookup.

Good to know

Two gallons, two mpg

The imperial gallon is 4.54609 L against the US 3.785411784 L, so a car rated 40 mpg (UK) manages only about 33 mpg (US). Comparing a British review with an American sticker without converting flatters the British car by a fifth.

Why L/100km flips the math

Consumption measures fuel per distance, economy measures distance per fuel. Doubling mpg halves L/100km rather than doubling it; which is why efficiency gains look smaller on the European scale at high mpg.

Reading real-world ratings

Official figures come from lab cycles; add roughly 10–20% consumption for mixed real driving. Converting a foreign-market rating before shopping for imports prevents unpleasant pump surprises.

How it's calculated

The math behind this calculator

L/100km = 235.215 ÷ mpg(US) = 282.481 ÷ mpg(UK) mpg(US) = 235.215 ÷ L/100km mpg(UK) = 282.481 ÷ L/100km

Miles-per-gallon and liters-per-100-km are inversely related; higher is better in one and worse in the other; so a direct ratio would be wrong. Every conversion routes through liters per 100 km using the exact constants above, which also handles the US versus UK gallon gap automatically.

Assumptions & limitations

  • Results are rounded only for display; full precision is used internally.
  • Conversions use exact standard definitions of each unit.

Worked example

A car rated at 25 mpg (US) consumes about 9.41 L/100km; the figure European spec sheets would print.

FAQ

Frequently asked questions

How do I convert 30 mpg (US) to L/100km?
Divide 235.215 by 30; about 7.84 L/100km. The calculator does this plus the UK-gallon variant automatically.
Is kpl (km per liter) supported?
Not directly, but kpl converts to L/100km by dividing 100 by kpl; convert that figure here for any other unit.
Why is my result lower after tuning?
If you switched scales (mpg → L/100km), lower is actually better; consumption fell. Check which direction the scale rewards before judging the tune-up.

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