MPG ↔ L/100km Inverse Converter (MPG Illusion)

For most of the twentieth century, fuel economy was nobody's problem. American cars averaged 14 MPG combined in 1973. Then the OPEC oil embargo hit, fuel prices quadrupled, and Congress responded with the Energy Policy and Conservation Act of 1975 - which created CAFE, the Corporate Average Fuel Economy standard. It was denominated, of course, in miles per gallon.

CAFE worked. New US passenger-car economy doubled from 14 MPG in 1973 to 27.5 MPG by 1985. But Larrick and Soll's 2008 paper would later show that the MPG metric itself was misleading regulators and consumers about where the biggest savings actually lived - in dragging the worst vehicles up from 10 to 15 MPG, not in pushing the best from 30 to 50.

Across the Atlantic, European countries were quietly adopting a different metric. West Germany's ADAC and France's Argus magazine had been using L/100km in their tests since the late 1970s. The 1999 EU Directive 1999/94/EC formalised it: every new-car advertisement, sales brochure, and showroom poster in EU member states from January 2001 had to display L/100km on the headline label. The reciprocal MPG was relegated to a small-print conversion line.

In June 2008, Richard Larrick and Jack Soll of Duke University's Fuqua School of Business published "The MPG Illusion" in Science. The paper used a simple example: replacing a 10 MPG truck with a 15 MPG truck saves more fuel over 10,000 miles than replacing a 34 MPG car with a 50 MPG car. The L/100km math reveals this instantly; the MPG math hides it because MPG compresses the high end and stretches the low end non-linearly. The paper directly influenced the EPA's 2007 decision to add gallons per 100 miles to the US window sticker.

European homologation testing went through its own revolution. NEDC (New European Driving Cycle), used from 1996 to 2017, was notoriously gentle - 20 minutes of low-load cycling with no air conditioning, lots of coasting, peak speed 120 km/h. Real-world fuel economy was 20-40 percent worse. The 2015 Dieselgate scandal accelerated regulatory anger, and in September 2017 WLTP (Worldwide Light-vehicles Test Procedure) replaced NEDC. WLTP is 30 minutes, peaks at 131 km/h, includes realistic acceleration. Reported L/100km figures jumped 20 percent overnight - same cars, more honest test.

The 2020s saw L/100km extend to EVs as kWh/100km. EU consumer labels list both - a Volkswagen ID.3 spec sheet reports 15 kWh/100km and 2 L/100km gasoline equivalent. The US EPA still favours MPGe (miles per gallon equivalent) and only sometimes lists kWh/100km in fine print. The Larrick illusion persists in the new units too.

By 2026, the EU has committed to phasing out new internal-combustion-engine passenger cars by 2035. L/100km will become a legacy unit referenced for cost-per-mile comparison against EVs. MPG will remain the US headline number until US consumer culture catches up with what European regulators decided in 1999. Our dual-needle gauge exists to bridge that gap - one minute of dragging tells the cross-Atlantic story.