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Convert Miles per Gallon UK to Kilometers per Liter - Fuel Economy Converter

This converter changes fuel economy expressed in miles per imperial gallon, commonly used in the United Kingdom, into kilometers per liter, commonly used in many metric jurisdictions. The conversion is a fixed mathematical relationship and is instantaneous.

Results assume the imperial gallon definition of 4.54609 liters and the international mile of 1.609344 kilometers. Use this tool to compare vehicle economy figures, report fleet metrics, or translate manufacturer ratings between unit systems.

Read the methodology and accuracy notes before using results for regulatory reporting or reimbursement calculations. For metrology and legal-traceability guidance consult the cited standards bodies.

Updated Nov 1, 2025

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Methodology

This conversion uses exact defined constants: 1 international mile = 1.609344 kilometers and 1 imperial gallon = 4.54609 liters. The kilometer per liter value is computed by multiplying miles per imperial gallon by the ratio of kilometers per mile divided by liters per imperial gallon.

Because the relationship is purely arithmetic with fixed constants, the converter is deterministic. Precision of displayed results depends on rounding choices; the underlying calculation retains full numeric precision from the constants above.

For regulatory, legal, or test-laboratory reporting verify instrument calibration and measurement traceability according to national metrology guidance. Where required, report values with the number of significant figures mandated by the receiving authority.

Worked examples

Example 1: 10 mpg (UK) → 10 * 0.354006 ≈ 3.5401 km/L.

Example 2: 50 mpg (UK) → 50 * 0.354006 ≈ 17.7003 km/L.

Example 3: 72.5 mpg (UK) → 72.5 * 0.354006 ≈ 25.6654 km/L.

Further resources

Expert Q&A

What is the difference between UK (imperial) mpg and US mpg?

UK mpg uses the imperial gallon of 4.54609 liters. US mpg uses the US liquid gallon of 3.785411784 liters. Converting from US mpg requires a different factor. This tool specifically converts imperial (UK) mpg to km/L.

How many decimal places should I report?

Choose precision based on use case. For simple comparisons two decimal places are usually sufficient. For regulatory or laboratory reporting follow the precision requirements specified by the receiving authority or test protocol.

Are converted values legally binding for emissions or tax reporting?

Conversion is a mathematical translation of units only. Legal acceptance depends on the regulating agency and whether measurements meet metrological traceability and instrument calibration requirements. Consult local regulators and follow standards and test procedures.

Why might my on-road fuel economy differ from manufacturer ratings?

Real-world economy varies with driving behavior, load, maintenance, tire pressure, fuel quality, and test conditions. Manufacturer figures are typically from controlled test cycles; use converted values only for unit translation, not to alter the underlying measurement conditions.

How accurate is the conversion factor?

The factor is derived from internationally defined unit values and is effectively exact for unit conversion. Measurement uncertainty arises from the accuracy of the reported mpg value, not from the conversion arithmetic. Follow metrology best practices for measurement uncertainty when reporting.

Sources & citations