Litres per 100 Kilometres to Miles per Gallon (US) (L/100km to mpg)

Litres per 100 kilometres (L/100 km) measures fuel consumption directly: the volume of fuel in litres a vehicle consumes per 100 kilometres of distance travelled. It is a "fuel-per-distance" unit — lower numbers mean better economy, the cleaner mathematical convention because consumption scales linearly with distance. A 5 L/100 km car uses exactly half the fuel per kilometre of a 10 L/100 km car, while in MPG the same physical halving moves the figure non-linearly along a hyperbolic curve. The "per 100 km" scaling produces typical passenger-car figures in the 4-12 range, comparable in legibility to MPG figures in the 20-50 range. The litre is exactly 1 cubic decimetre (1 dm³) by SI definition; the kilometre is exactly 1000 metres.

Litres per 100 kilometres emerged as the European fuel-economy convention in the 1970s after most of continental Europe completed metrication in the 1960s and needed a consumption-rather-than-distance-based unit that worked cleanly with the SI litre and the kilometre. France and West Germany were early adopters, with the unit becoming the European norm by the 1980s and codified in Council Directive 80/1268/EEC, which established the New European Driving Cycle (NEDC) test procedure used until 2017. The Worldwide Harmonised Light Vehicles Test Procedure (WLTP) replaced NEDC in September 2017 to better reflect real-world driving, with an additional Real Driving Emissions (RDE) test for nitrogen oxides; both regimes report headline figures in L/100 km. Canada switched its primary fuel-economy reporting to L/100 km in 1981 under Transport Canada's EnerGuide programme, joining the European convention even though the United States retained the inverted MPG unit. Australia's Green Vehicle Guide adopted L/100 km in 2001, the UK's official VCA reporting switched in 2017 alongside WLTP, and the European Commission's 1999/94/EC fuel-economy labelling directive (in force since January 2001) requires every new-car showroom display in the EU to publish the L/100 km figure prominently. The "per 100 km" denominator was chosen so typical passenger-car figures fall in a legible single-digit range (4-12) rather than the awkward fractional figures a per-kilometre denominator would produce.

L/100 km is the primary fuel-economy unit on every new-car efficiency sticker sold in the European Union, the United Kingdom, Switzerland, Norway, Iceland, Australia, New Zealand, and Canada. The European Commission's mandatory fuel-economy and CO2 labelling regulation (Directive 1999/94/EC) requires the L/100 km figure on every new-car showroom display in the EU. Canadian Transport Canada's EnerGuide labels publish L/100 km figures alongside an "annual fuel cost" estimate. Australian Greenhouse Office's Green Vehicle Guide reports L/100 km against a CO2-emissions class. Most automotive trade press outside the United States — Auto Express UK, Auto Bild Germany, l'Argus France, Wheels Australia — reports fuel economy in L/100 km regardless of host-country MPG conventions. EU fleet-CO2 regulation (Regulation 443/2009 superseded by 2019/631) sets manufacturer-fleet-average targets in g CO2/km, derived from L/100 km via the carbon density of petrol or diesel.

One mile per US gallon (mpg) is the distance in statute miles a vehicle travels per US gallon of fuel consumed. It is a "distance-per-fuel" unit — higher numbers mean better economy, the inverse of the European L/100 km convention where lower numbers are better. The MPG figure on every EPA window sticker is reported separately for city driving, highway driving, and a combined-cycle average derived from a weighted blend of the two test cycles, with the test methodology specified in 40 CFR Part 600. The US gallon used is exactly 3.785411784 litres, distinct from the UK imperial gallon of 4.546 litres, which produces a different MPG figure (~20% higher) for the same physical efficiency. The mile is the international statute mile of exactly 1609.344 metres.

The miles-per-gallon convention emerged in the United States in the early 20th century, when automobile makers and the petroleum industry needed a consumer-facing efficiency metric for an economy where road distances were measured in miles and fuel was sold by the US gallon. Early road tests in publications like Motor Age and Automotive Industries reported MPG figures from the 1910s onward, and by the postwar boom in private car ownership the unit was a fixed feature of new-car advertising and Consumer Reports test methodology. The 1973 oil shock and the Energy Policy and Conservation Act of 1975 elevated MPG to a federal regulatory unit: the Environmental Protection Agency was charged with publishing fuel-economy ratings on every new-car window sticker, and the Corporate Average Fuel Economy (CAFE) standards set fleetwide MPG targets that automakers had to meet under threat of civil penalty. The window-sticker methodology has been recalibrated several times — most notably the 2008 introduction of "five-cycle" testing that accounted for cold-start, air-conditioning, and high-speed driving — but the headline mile-per-US-gallon figure on every American new-car window sticker has remained the dominant fuel-economy unit in US consumer measurement since the late 1970s. The 2007 Energy Independence and Security Act tightened CAFE standards to a 35 mpg fleet average by 2020, and the 2012 EPA-NHTSA joint rulemaking pushed light-truck and passenger-car CAFE targets toward 54.5 mpg by model year 2025 before the 2020 SAFE Vehicles Rule rolled them back.

MPG is the dominant fuel-economy unit on every US new-car window sticker, every entry in the EPA's fueleconomy.gov database, every US automotive trade-press review, and every CAFE-compliance filing by US automakers. UK car magazines and government MOT documentation report MPG against the imperial gallon, producing a different headline figure for the same physical efficiency — a US-spec 30 mpg vehicle is rated at 36 mpg on UK measurement, the source of frequent cross-Atlantic confusion in import buying. Canadian Transport Canada moved primary reporting to L/100 km in 1981 but informal Atlantic-Canadian car-buyer speech retains imperial-gallon MPG. Most other markets — continental Europe, Australia, New Zealand, Japan, India — use L/100 km or kilometres per litre instead. US fleet operators (rental car companies, delivery fleets, government fleets) track MPG as a primary cost-management metric.