# Fuel Economy Converter — MPG, L/100km, km/L & More > Convert between mpg (US & UK), L/100km, km/L and mi/L instantly. Understand the inverse relationship between mpg and litres-per-100km with a visual comparison bar. Free and instant. - **Category:** Conversions & Units - **Interactive calculator:** https://youcalc.com/en/conversions-units/fuel-economy-converter/ - **Price:** Free, no sign-up required ## Overview This converter translates a fuel-economy figure between the five units used around the world: mpg (US gallon), mpg (Imperial/UK gallon), litres per 100 kilometres, kilometres per litre, and miles per litre. Use it to compare a European car spec sheet with a North American one, check whether your vehicle meets a fuel-economy standard stated in unfamiliar units, or simply satisfy curiosity about what your fuel figure means in another system. ## How to read your result Type a value in any unit and every other field updates instantly. The key insight is that L/100km is an inverse efficiency measure — a lower number is better (you burn fewer litres), while mpg and km/L are direct measures where a higher number is better. The visual comparison bar makes that inversion visible at a glance. US and Imperial mpg differ because the US gallon (3.785 L) is smaller than the Imperial gallon (4.546 L), so the same car always shows a higher mpg number on the Imperial scale. ## Method Every input is first converted to L/100km as an internal pivot, then fanned out to the remaining four units. For mpg (US): L/100km = 235.215 ÷ mpg(US); for mpg (UK): L/100km = 282.481 ÷ mpg(UK); for km/L: L/100km = 100 ÷ km/L; for mi/L: convert to km/L by multiplying by 1.609 344, then invert as above. From L/100km the other units follow: km/L = 100 ÷ L/100km, mpg(US) = 235.215 ÷ L/100km, mpg(UK) = 282.481 ÷ L/100km, mi/L = km/L ÷ 1.609 344. L/100km is chosen as the pivot precisely because its inverse relationship with mpg is the main conceptual challenge this calculator is designed to clarify. The US gallon is smaller than the Imperial gallon, so the same car always shows a higher mpg on the Imperial scale; the converter makes this difference explicit by computing both simultaneously. ## Example - **Setup:** Enter 30 in the mpg (US) field. - **Result:** The converter returns 7.84 L/100km, 36.03 mpg (UK), 12.75 km/L, and 7.93 mi/L. Notice that 30 US mpg and 36 UK mpg describe the exact same fuel efficiency — the difference is entirely due to the gallon size. ## Frequently asked questions ### Why is US mpg always lower than UK mpg for the same car? The US gallon holds 3.785 litres while the Imperial gallon holds 4.546 litres — about 20% more. Because the Imperial gallon is larger, a car travels more miles per Imperial gallon than per US gallon, so UK mpg figures are always about 20% higher for the same vehicle. Neither number is 'wrong'; they simply use different gallon definitions. ### Why does a lower L/100km mean better efficiency? L/100km measures fuel consumed per fixed distance — it asks 'how many litres does it take to cover 100 km?' Less fuel for the same distance means better efficiency, so lower is better. mpg and km/L ask the opposite question — 'how far can I go on one unit of fuel?' — so higher is better for those. Because L/100km is the reciprocal of km/L, doubling your km/L halves your L/100km. ### Which unit should I use? Use L/100km if you are in continental Europe, Australia, or comparing against EU regulatory figures. Use mpg (US) for North American specs, and mpg (UK) for British vehicles or insurance documents from the United Kingdom. km/L is common in Japan, India, and South-East Asia. mi/L is rare but occasionally appears in older UK engineering references. ### Does the converter account for actual driving conditions? No — this is a pure unit conversion. It accurately translates a given efficiency figure from one unit to another, but it does not model real-world variables such as driving style, speed, load, tyre pressure, temperature, or fuel grade. Always compare figures measured under the same test cycle (for example, WLTP vs EPA) before drawing conclusions. ### What are the exact conversion constants used? The calculator uses NIST SP-811 exact definitions: 1 US gallon = 3.785 411 784 L, 1 Imperial gallon = 4.546 09 L, 1 mile = 1.609 344 km. These give pivot constants of 235.215 (L·100km⁻¹ × mpgUS) and 282.481 (L·100km⁻¹ × mpgUK). All intermediate rounding is deferred until final display. ## Related calculators - [Unit Converter](https://youcalc.com/en/conversions-units/unit-converter/) - [Temperature Converter](https://youcalc.com/en/conversions-units/temperature-converter/) - [Land Area Converter](https://youcalc.com/en/conversions-units/land-area-converter/) - [Recipe Converter](https://youcalc.com/en/cooking-food/recipe-converter/) ## Sources - https://www.fueleconomy.gov/feg/info.shtml - https://www.nist.gov/pml/special-publication-811 --- Interactive version: https://youcalc.com/en/conversions-units/fuel-economy-converter/ · From YouCalc — https://youcalc.com