# Fuel Economy Converter

- **URL:** https://codeasystem.com/calculators/conversion/fuel-economy-converter/
- **Category:** conversion
- **Description:** Convert fuel consumption between US mpg, UK mpg and liters per 100 km with exact triangular formulas; the mpg ↔ L/100km bridge.
- **Primary output:** Conversion: 25 mpg (US) = 9.4086 L/100km

## Inputs
- Value (name: `value`, type: number, example: 1)
- From (name: `from`, type: select)
- To (name: `to`, type: select)

## Outputs
- Conversion: 25 mpg (US) = 9.4086 L/100km
- In mpg (UK): 30.0237

## Formula / methodology
```
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.

## How to use
1. **Enter the rating**; Use the number from the window sticker or brochure.
2. **Say which gallon**; US and UK mpg differ by about 20%; pick the right source country.
3. **Compare all three**; The result shows both remaining units so nothing needs a second lookup.

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

Result for these inputs:

```
Conversion: 25 mpg (US) = 9.4086 L/100km
```

## About this calculator
### 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.

## FAQs
### 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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Last updated: 2026-08-23 · Version: 1.0.0 · [HTML version](https://codeasystem.com/calculators/conversion/fuel-economy-converter/)