Fuel efficiency relates the efficiency of conversion to kinetic energy from energy contained in a carrier fuel, specifically in a transportation vehicle, such as an automobile.

It should be noted that despite common usage, "fuel efficiency" is not a synonym for "fuel economy" or "gas mileage". This is significant because the improvements in fuel efficiency achieve over the last 20 years have NOT been translated into improvements in fuel economy--much of the savings have been offset by SUVs and power-enhancing improvements. e.g., Hummers may have higher fuel efficiency to a small car, but have equal or lower fuel economy. (This unintended loophole is the result of unclear legislative wording in CAFE standards.)

The measure is usually expressed in one of two ways:

1. As the amount of fuel used per unit distance; for example, litres per 100 kilometres (L/100km). In this case, the lower the value, the more efficient a vehicle is;
2. As the distance travelled per unit volume of fuel used; for example, kilometres per litre (km/L or kpl) or miles per gallon (mpg). In this case, the higher the value, the more efficient a vehicle is.

The two european standard measuring scenarios for "L/100km" value are autobahn travel at 90km/h and rush hour city traffic. A reasonably modern european subcompact car may manage highway travel at 5 litres per 100 kilometers or 6,5 litres in city traffic, with app. 140 gramms of CO2 emission per km.

An average "car-shaped" US car produces circa 27 mpg, a large SUV usually gets 15 mpg.

The driving interval tests here described test emissions, and MAYBE fuel economy, but certainly not fuel efficiency.

In the United States, the Environmental Protection Agency (EPA) is the government body that makes the calculations that auto manufacturers use when advertising their vehicles. Separate numbers are given for city and highway driving. The EPA tests do not directly measure fuel consumption, but rather calculate the amount of fuel used by measuring emissions from the tailpipe based on a formula created in 1972. The cars are not actually driven around a course, but are cycled through specific profiles of starts, stops, and runs on a dynamometer in a laboratory environment. As emissions standards have become more strict due to smog, some of the resulting numbers do not directly correspond to what people actually experience when driving. Most often, the EPA estimate of mileage is several percent higher than what the average driver manages.

Here are some common conversion factors:

To convert x L/100 km to y MPG, perform:

• 235.2146 ÷ x L/100km = y MPG (US liquid gallon), or
• 282.481 ÷ x L/100km = y MPG (Imperial gallon)

To convert a MPG to b L/100km, perform:

• 235.2146 ÷ a MPG (US liquid gallon) = b L/100km, or
• 282.481 ÷ a MPG (Imperial gallon) = b L/100km

To convert m km/L to n MPG, perform:

• 2.352146 * m km/L = n MPG (US liquid gallon), or
• 2.82481 * m km/L = n MPG (Imperial gallon)

To convert c MPG to d km/L, perform:

• 0.4251437 * c MPG (US liquid gallon) = d km/L, or
• 0.354006 * c MPG (Imperial gallon) = d km/L

Tips

• Make sure air pressure in your car's tires is correct.
• Avoid abrupt acceleration and deceleration; try to keep a stable speed and coast whenever possible.
• If driving a car with a manual transmission, shift up as early as reasonably possible, and shift down late
• Make sure your car's engine is well tuned.
• Do not carry unnecessary loads in the car.
• Driving at high speeds with the windows open creates a lot of aerodynamic drag, which lowers fuel efficiency.
• Do not let the engine idle unnecessarily. Unless in traffic, shut it down whenever it is unused for more than 10 seconds.

See also

• Corporate Average Fuel Economy
• Emission standard
• EPA 2004 fuel economy report appendix M2

External links

• Fuel saving synthetic lubricants
• EPA Green Vehicle Guide
• Canadian Energuide: Vehicles
• FuelEconomy.gov