Fuel Economy Conversion Guide

Fuel economy is the rate at which a vehicle converts fuel into distance — distance per unit fuel (mpg, km/L) or fuel per unit distance (L/100km). The two forms are mathematical reciprocals: km/L = 100 / (L/100km). Both forms survived because each is more intuitive for a different question. Distance per fuel answers 'how far can I drive on a tank'. Fuel per distance answers 'how much fuel will this trip take'.

The reciprocal relationship breaks linear intuition. Drivers who assume 'doubling mpg halves fuel use' are right in any single comparison, but the savings are not uniform along the scale. Replacing a 12 mpg pickup with a 24 mpg sedan saves twice as much fuel over the same distance as replacing a 30 mpg sedan with a 60 mpg hybrid, even though both replacements double the mpg figure.

The US adopted miles per gallon in the early 20th century, following the imperial origin of both the mile and the gallon. The Energy Policy and Conservation Act of 1975 codified Corporate Average Fuel Economy (CAFE) standards for US automakers, measured in mpg. The EPA published the first fuel-economy guide for consumers in 1977. Test procedures have evolved several times — the 1985 'EPA combined' rating, the 2008 '5-cycle' adjustment that lowered ratings by 10–25% to better match real driving, and the ongoing argument about plug-in hybrid certification.

Europe and most of the rest of the world moved to litres per 100 kilometres in the metric transition. The L/100km figure is the inverse of km/L and matches the European driver's intuition that fuel is the budget and distance is the trip length. The current EU procedure is WLTP (Worldwide Harmonised Light Vehicle Test Procedure), which replaced the optimistic NEDC test in 2017 and brought published figures closer to real-world driving.

The UK kept the imperial gallon through the metric transition for fuel — a UK 'gallon' is 4.54609 L exactly, versus the US gallon of 3.785411784 L. A UK car rated at '40 mpg' achieves the same fuel efficiency as a US car rated at '33 mpg', because the gallon is 20% larger. The EPA and the UK Department for Transport publish ratings in their respective gallons, and crossing the Atlantic without converting leads to systematic over- or underestimates.

Electric vehicles broke the gallon-based mpg system, and the EPA responded with MPGe (miles per gallon gasoline equivalent). One gallon of petrol stores about 33.7 kWh of usable energy by EPA convention. An EV consuming 30 kWh per 100 miles is therefore equivalent to one gallon (33.7 kWh) per 112 miles, or 112 MPGe. The MPGe figure lets buyers compare an electric Tesla to a petrol Camry on a common scale, with the caveat that 'gallon equivalent' depends on the EPA's chosen energy-per-gallon factor and ignores grid generation efficiency.

Our fuel-economy category supports four units. SI metric: kilometre per litre (km/L, our base unit). European convention: litre per 100 km (L/100km, the reciprocal form). US imperial: mile per gallon US (mpg US). UK imperial: mile per gallon UK (mpg UK). Each declares its km/L equivalent: 1 mpg US = 0.425144 km/L, 1 mpg UK = 0.354006 km/L, and L/100km is the reciprocal form handled with the (value === 0 ? 0 : 100 / value) bridge.

We do not include MPGe as a separate unit because it is electric-vehicle-specific and depends on the EPA's 33.7 kWh/gallon convention. If you need MPGe, multiply your kWh/100 mi figure by 33.7 to get gallon-equivalents per 100 mi, then take the reciprocal for the MPGe number.

Fuel economy is one of two categories on Calcflux that route through a non-linear bridge — L/100km is the reciprocal of km/L (100 ÷ value), not a multiplicative factor. Every other unit reduces to km/L through a single multiplicative factor. The methodology page lists the explicit formula for each unit and notes the reciprocal handling.

Useful round-number conversions: 1 US mpg ≈ 0.425 km/L ≈ 235 / (mpg) L/100km. 1 UK mpg ≈ 0.354 km/L ≈ 282 / (mpg) L/100km. 1 L/100km ≈ 100 km/L (reciprocal). For sanity checks: a 30 US mpg car uses about 7.8 L/100km; a 40 US mpg hybrid uses 5.9 L/100km; a 25 UK mpg classic uses 11.3 L/100km. The EPA fuel-economy site at fueleconomy.gov publishes the same calculations with worked examples.

Cross-border car shopping and travel drive most fuel-economy searches. Miles per gallon US to litres per 100 km and litres per 100 km to miles per gallon US handle the common US-Europe comparison. A 30 US mpg car is 7.8 L/100km; a 6 L/100km European compact is about 39 US mpg. Kilometres per litre to miles per gallon US covers the metric-decimal form used in some Asian markets. Miles per gallon US to kilometres per litre is the inverse for converting US ratings into the km/L convention.

The full fuel-economy comparison table lists every supported unit with its km/L factor and a worked sample. Use it when you need to convert a UK road-test rating into US mpg or into L/100km for a European fuel-budget plan.

Reciprocal, not linear: mpg is the reciprocal of fuel use, so changes near the low end of the scale save more fuel than equal-sized changes near the high end. Replacing a 12 mpg vehicle with one at 24 mpg saves 416 gallons over 10,000 miles; replacing a 30 mpg vehicle with one at 60 mpg saves only 167 gallons. EPA marketing learned this lesson the hard way, and now publishes 'gallons per 100 miles' alongside mpg on US window stickers specifically to make the reciprocal-vs-linear comparison clearer.

US vs UK gallon: a US gallon is 3.785411784 L; a UK gallon is 4.54609 L. UK mpg ratings are 20% higher than US mpg ratings for the same vehicle. A car advertised at '50 mpg' in a UK road test would be rated about '42 mpg' on a US window sticker. Cross-Atlantic price comparisons that ignore this lead to consistent overestimates of UK car efficiency.

L/100km is the inverse, not a parallel: a smaller L/100km figure is a more efficient car, the opposite of mpg's larger-is-better convention. New drivers from mpg countries sometimes read European reviews backwards. Anchor: 5 L/100km ≈ 47 US mpg; 10 L/100km ≈ 24 US mpg; 20 L/100km ≈ 12 US mpg.

EPA test cycle changes: the EPA combined rating was lowered by 10–25% in 2008 because the old test cycle was unrealistically gentle. A 1995 car rated '30 mpg' under the old test would be rated '24 mpg' under the new one. Comparing historical ratings to current window-sticker numbers without the cycle adjustment is misleading.

MPGe and grid efficiency: an EV rated '120 MPGe' uses 33.7 kWh per 120 miles, which the EPA calls 'one gallon equivalent'. The figure ignores upstream power-plant efficiency and transmission losses — well-to-wheels analysis lowers EV efficiency relative to MPGe by roughly a factor of two depending on the grid mix. MPGe is a fair fuel-equivalent comparison; it is not a primary-energy comparison.

Side-by-side: the full fuel-economy comparison table lists every supported unit with its factor and a worked sample. Method and sources: the fuel-economy section of our methodology page cites the EPA fueleconomy.gov reference. The EU WLTP procedure governs European published mpg/L/100km figures since 2017 but is a vehicle test cycle, not a unit definition. Context: our piece on fuel-economy ratings explained walks through the EPA test cycles, the US/UK gallon split, and MPGe with worked examples.