Units of measure

The two most common ways to measure automobile fuel economy are:

1. The amount of fuel used per unit distance; most commonly, litres per 100 kilometres (l/100 km). Lower values mean better fuel economy: you use less fuel to travel the same distance.
2. The distance travelled per unit of fuel used; most commonly kilometres per litre (km/l) or miles per gallon (mpg), which may be U.S. gallon or imperial gallon, although the imperial gallon is about 20% larger than the U.S. gallon. Higher values mean better fuel economy: you can travel farther for the same amount of fuel.

To convert between l/100 km and miles per U.S. gallon, divide 235 by the number in question. For miles per imperial gallon, use 282 instead of 235. For example, to convert from 30 mpg (U.S.) to l/100 km, divide 235 by 30, giving 7.83 l/100 km; or from 10 l/100 km to mpg U.S., divide 235 by 10 (23.5 mpg). To convert from l/100 km to km/l, divide between 100 and calculate the reciprocal of the result.

A related measure is the amount of carbon dioxide produced as a result of the combustion process, typically measured in grams of CO₂ per kilometre (CO₂ g/km). A petrol (gasoline) engine will produce around 2.32 kg of carbon dioxide for each litre of petrol consumed (19.4 lb/gal). A typical diesel engine produces 2.66 kg/l (22.23 lb/gal) though typically burns fewer litres per kilometre (and is thus typically more fuel efficient for an otherwise identical car). Since the CO₂ emissions are relatively constant per litre, fuel efficiency is directly related to emissions of CO₂ per kilometre.

Inverse scale

A modest improvement in fuel economy for a relatively inefficient vehicle can provide greater savings in terms of financial cost to the driver and environmental impact than a proportionately larger increase for a more economical vehicle. This is most intuitively demonstrated using the inverse scale - gallons per mile or liters per kilometer. If a driver who travels 15,000 miles a year switches from a vehicle with 10 mpg to 12 mpg average fuel economy (0.10 gallons per mile to 0.083 gallons per mile), 250 gallons are saved. A similar 20% improvement in exchanging a 30 mpg for a 36 mpg (0.033 gallons per mile for 0.27) vehicle saves only 83 gallons for similar driving patterns.

Fuel economy statistics

The choice of car and how it is driven drastically affects the fuel economy. A top fuel dragster can consume 6 U.S. gallons (23 L) of nitromethane for a quarter-mile (400 m) run in about 4.5 seconds, which comes out to 24 U.S. gallons per mile (5600 L per 100 km). The other extreme was set by a French entrant in the Eco-Marathon in 2004, who managed 3410 km/l, or more than 8000 mpg-U.S. .