Fuel efficiency

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(Redirected from Fuel economy)

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. Fuel economy relates to the amount of fuel required to move a vehicle over a given distance.


Fuel efficiency

Fuel efficiency is normally expressed in terms of power per unit of engine displacement, also known as specific output. In the United States, horsepower per liter is a common metric. It should be noted that despite common usage, "fuel efficiency" is not a synonym for "fuel economy" or "gas mileage". Modern fuel injected engines are much more efficient at producing power than their carbureted predecessors. For example, power output from Chrysler's 3.9 L LA V6 engine jumped from 125 hp (93 kW) to 180 hp (134 kW) in 1992 due to the addition of fuel injection and a freer-flowing intake manifold.

However, 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 the use of heavier and less-aerodynamic body styles (especially SUVs and pickup trucks) and the use of more-powerful engines. For example, the 6.0 L Vortec V8 used in the Hummer H2 produces 53.6 hp (39 kW) per liter of displacement, which is more than double the 25.4 hp (19 kW) per liter produced by the original VW Beetle. However, the Hummer weighs more than three times as much as the Beetle, has a much less-aerodynamic body, and uses a complex four wheel drive system, so the Beetle is able to travel three times farther than the Hummer on the same amount of fuel.

Fuel economy

Fuel economy 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/100 km). 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 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/100 km" value are autobahn travel at 90 km/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 grams of carbon dioxide emission per km.

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

The driving interval tests described here test emissions and 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/100 km = y MPG (US liquid gallon), or
  • 282.481 ÷ x L/100 km = y MPG (Imperial gallon)

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

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

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


  • Make sure air pressure in your car's tires is correct.
  • Avoid abrupt acceleration and deceleration; accelerate as gradually as possible (especially uphill), try to keep a stable speed, time your movement to minimize slowing (especially complete stops), and coast whenever possible.
  • If driving a car with a manual transmission use the highest reasonable gear; shift up early 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.
  • Use air conditioning sparingly and try setting the temperature control higher. It is estimated that the air conditioning running at "Max" setting can lower fuel efficiency by 5 to 25%. If travelling at high speeds, however, the drag due to an open window may consume more fuel than operating the AC sensibly.
  • Avoid driving at excessively low or high speeds. Most gasoline powered cars operate with maximum efficiency in the range of 45 to 60 mph ( 70 to 100 km/h ). This effect is largely due to aerodynamic drag. For vehicles with greater frontal area and less streamlined shapes (SUVs, trucks) best fuel economy is typically obtained toward the lower end of this range.

See also

External links


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