In automobiles, fuel efficiency refers to energy efficiency that consumes fuel. It is usually expressed as the distance that can be driven per unit of fuel. The fuel efficiency notation method differs by country and region, and each country's government prepares official fuel economy regulations and obligates consumers to notify them. In most countries, including Korea, it is expressed in km (km/ℓ, kilometers per liter) per liter, and in the United States, it is expressed in miles per gallon (mpg), that is, the distance that can be driven in 1 gallon. In some countries such as Europe and Oceania, the amount of fuel that can drive 100km (nℓ /100km) is expressed as fuel economy. In addition, it is common to record the emission of carbon dioxide (CO₂) together with fuel economy. Most measure carbon dioxide emissions by collecting exhaust gas generated during simulated driving for fuel economy measurement. The resulting value is expressed as the amount of carbon dioxide emitted when driving every 1km.