The US and UK measure fuel economy in miles per gallon (MPG) — higher is better. Europe uses liters per 100 kilometers (L/100km) — lower is better. These inverse scales create a psychological trap that misleads car buyers about the value of efficiency improvements.
The MPG Illusion
Consider two cars: Car A improves from 20 to 25 MPG. Car B improves from 40 to 50 MPG. Which saves more fuel? Most people say Car B (bigger MPG gain), but Car A saves significantly more.
Driving 10,000 miles/year: Car A (20→25 MPG) uses 500→400 gallons = saves 100 gallons. Car B (40→50 MPG) uses 250→200 gallons = saves 50 gallons. Car A saves twice as much fuel despite a smaller MPG improvement.
This is the "MPG illusion" — linear MPG improvements don't translate to linear fuel savings. L/100km doesn't have this problem because it's already proportional to fuel consumption.
Converting Between Systems
US MPG to L/100km: L/100km = 235.2 ÷ MPG. So 30 MPG = 7.84 L/100km.
UK MPG to L/100km: L/100km = 282.5 ÷ MPG. UK gallons are larger, so UK MPG numbers are higher. 40 UK MPG = 7.06 L/100km.
Quick reference: 20 MPG ≈ 11.8 L/100km, 30 MPG ≈ 7.8 L/100km, 40 MPG ≈ 5.9 L/100km, 50 MPG ≈ 4.7 L/100km.
Global Fuel Economy Standards
Corporate Average Fuel Economy (CAFE) standards in the US require ~40 MPG average by 2025. EU standards require 95 g CO₂/km ≈ 4.1 L/100km (gasoline) by 2024.
Electric vehicles use different metrics: kWh/100km (metric) or MPGe (US). 1 gallon of gasoline = 33.7 kWh of electricity. A Tesla Model 3 at 15 kWh/100km = 112 MPGe.
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