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Convert Miles per Gallon UK to Gallons per 100 Miles - Fuel Economy Converter

This converter transforms fuel economy stated as miles per imperial gallon (mpg, UK) into imperial gallons consumed per 100 miles. The output is the volume of imperial gallons required to travel 100 miles given the entered mpg value.

The operation is a fixed mathematical inversion and multiplication. Enter one numeric mpg (UK) value and the tool returns the corresponding gallons per 100 miles for the imperial gallon definition used in the United Kingdom and related jurisdictions.

Use this for quick comparisons, reporting or to translate consumer mpg figures into a consumption-per-distance metric commonly used in fleet management and policy analysis. See methodology and caveats for precision and measurement guidance.

Updated Nov 22, 2025

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Methodology

The conversion uses the definition miles per gallon (mpg) where gallon refers to the imperial gallon (exactly 4.54609 litres). The converter treats both input and output as using the same imperial gallon unit so no additional volumetric conversion is needed.

Because the relationship is algebraically exact, the tool computes the reciprocal of mpg and scales to a 100 mile basis. This is an arithmetic conversion and does not attempt to model driving cycles, measurement errors, or the effects of fuel temperature and density.

For traceability and unit notation the tool follows guidance from established standards for units and measurement practices. See the citations for references to unit conventions and laboratory accuracy standards.

Worked examples

Example 1: Input 40 mpg (UK); 100 divided by 40 equals 2.5 imperial gallons per 100 miles.

Example 2: Input 60 mpg (UK); 100 divided by 60 equals about 1.6666667 imperial gallons per 100 miles. Display rounding to two or three decimals depending on settings.

Example 3: Input 0.5 mpg (UK); 100 divided by 0.5 equals 200 imperial gallons per 100 miles (valid arithmetic but indicates extreme fuel usage).

Expert Q&A

Does this convert United States mpg to gallons per 100 miles?

No. This converter assumes the input is miles per imperial gallon (mpg UK). United States gallons are a different volumetric definition. To convert from US mpg you must first convert US gallons to imperial gallons using their exact litre definitions, or use a converter specifically for US mpg to gallons per 100 miles.

How should I interpret the result for reporting or regulation?

The numeric result is a direct mathematical conversion and not a certified laboratory measurement. For regulatory reporting refer to the applicable test procedure or certification standard (for example WLTP or local regulatory test cycles). Use the converted number to compare fuel economy metrics consistently, and document the unit (imperial gallons per 100 miles).

What precision should I use when reporting?

Choose precision that reflects measurement uncertainty. For typical consumer use display one or two decimal places. For technical or regulatory use follow the precision and rounding rules specified by the relevant standard or testing protocol and report uncertainty where required.

What input values are invalid?

Zero or negative mpg values are invalid because mpg is defined as distance per positive volume. Extremely small mpg values produce very large gallons per 100 miles and may indicate erroneous input or data collection error.

Does fuel temperature or density affect this conversion?

The arithmetic conversion itself is independent of temperature or density, but real-world fuel volume and energy content vary with temperature and composition. For metrological or commercial transactions apply the appropriate volume correction and density references as specified in measurement standards.

How does this tool handle rounding and display?

The tool calculates the exact arithmetic result and then applies user-facing rounding for display. For reporting retain an appropriate number of significant figures and document the rounding rule used.

Sources & citations