Fuel Economy Converter Widget

Add a fuel economy converter to your website. Readers enter a figure in MPG (US), MPG (UK), L/100 km, km per litre or miles per litre and see it in every other unit, so car reviews from any country can be compared directly.

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How it works

MPG and km per litre measure distance per unit of fuel (higher is better), while L/100 km measures fuel per distance (lower is better), so converting between them is a reciprocal, not a multiplication. The widget converts every input to kilometres per litre using the legal definitions - the international mile and the US gallon as NIST lists them, and the imperial gallon as the UK Weights and Measures Act 1985 defines it - then to each other unit. It also shows how much fuel 100 miles takes in litres and US gallons, which is the figure that actually scales with cost. A practical consequence of the reciprocal: improving a thirsty car from 10 to 20 MPG saves far more fuel over 100 miles than improving an efficient one from 40 to 50.

Method

  • 1 mile = 1.609344 km; 1 US gallon = 3.785411784 L; 1 UK gallon = 4.54609 L
  • km/L = MPG(US) x 1.609344 / 3.785411784 = MPG(UK) x 1.609344 / 4.54609
  • L/100 km = 100 / (km/L); so L/100 km = 235.2146 / MPG(US) = 282.4809 / MPG(UK)
  • MPG(UK) = MPG(US) x 1.200950; results shown to 2 decimal places

Worked examples

A US review figure

Inputs: 30 MPG (US)

Result: 36.03 MPG (UK), 7.84 L/100 km, 12.75 km/L, 7.93 miles per litre; 12.62 L (3.33 US gal) per 100 miles

30 x 1.609344 / 3.785411784 = 12.754 km/L; 100 / 12.754 = 7.84.

A European test figure

Inputs: 5 L/100 km

Result: 47.04 MPG (US), 56.5 MPG (UK), 20 km/L

235.2146 / 5 = 47.04; 282.4809 / 5 = 56.50.

Limitations

  • Converts units only; official figures from different test cycles (EPA, WLTP, older NEDC) are not comparable even after conversion.
  • Does not cover electric-vehicle units such as kWh/100 km or MPGe.
  • Official plug-in hybrid figures assume a set share of battery driving, so converting them says little about fuel use for a driver who rarely plugs in.

Where publishers use it

  • Car review sites with readers in several countries
  • Motoring forums comparing European and US test figures
  • Import and export car dealers
  • Fleet managers standardising reports

Questions

What is 30 MPG in L/100 km?

30 MPG (US) is 7.84 L/100 km; 30 MPG (UK) is 9.42 L/100 km.

Why is the conversion not a simple multiplication?

L/100 km is fuel per distance and MPG is distance per fuel, so one is the reciprocal of the other: halving L/100 km doubles MPG.

Why do UK and US MPG differ for the same car?

The imperial gallon is about 20% larger than the US gallon (4.54609 L against 3.785411784 L), so a car goes further on one.

Is 10 to 20 MPG really a bigger saving than 40 to 50?

Yes. Over 100 miles, 10 MPG uses 10 gallons and 20 MPG uses 5, saving 5 gallons; 40 MPG uses 2.5 and 50 MPG uses 2, saving half a gallon. The 'fuel for 100 miles' row makes this visible.

Is anything rounded?

Only the display, to two decimal places. The factors are the exact legal definitions.

Sources

  1. NIST Guide to the SI, Appendix B.8 - conversion factors - National Institute of Standards and Technology (SP 811) . Mile (international) and US gallon
  2. Weights and Measures Act 1985, Schedule 1 - UK legislation (legislation.gov.uk) . Imperial gallon = 4.54609 L

Cite or recommend this tool

If you reference this tool in an article, course or documentation, these formats are ready to copy. They are optional - nothing is added to your site unless you paste it.

A2Z Tools Fuel Economy Converter
https://a2z.tools/embed/fuel-economy-converter

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