Fuel efficiency

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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

Image:Small efficient cars in the Netherlands.jpeg Fuel efficiency is normally expressed in terms of power per unit of engine displacement, also known as specific output. 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 achieved 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 four times as much as the original 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.

Turbocharged engines tend to obtain the most fuel efficiency, with the Mitsubishi Lancer Evolution VIII MR FQ-400 2L straight-four engine having 200hp/L. It is a rare occurrence for a naturally aspirated engine to obtain a high fuel efficiency.

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 (47 mpg US) or 6.5 litres in city traffic (36 mpg US), with app. 140 grams of carbon dioxide emission per km.

An average "car-shaped" US car produces circa 27 mpg (US) highway, 21 mpg (US) city; a large SUV usually gets 13 mpg (US) city, 16 mpg (US) highway. Pickup trucks vary considerably; whereas a light US pickup with a 4 cylinder engine produces circa 28 mpg, a full-size US pickup with extended cab with an 8 cylinder engine produces circa 13 mpg (US) city, 15 mpg (US) highway.

All these previously-cited fuel economy values are for operation on petrol (gasoline.) New US light vehicles designated as flexible fuel vehicles (FFVs) running on E85 (85% ethanol, 15% gasoline) will typically achieve from 5% to 15% less fuel economy in mpg on pure E85 than when operated on pure gasoline. Older non-turbo-charged fuel-injected FFVs running on E85 will typically achieve about 25% to 30% less fuel economy on E85. Over 4 million FFVs are currently operated on US roadways as of 2005; most tend to be light trucks or van vehicles, although newer "car-shaped" high performance cars are also being introduced in the 2006 model year (e.g., 2006 GM Chevrolet Impala).

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 to achieve in practice, although there are some cases where the difference is nearly 200% higher than what the average driver achieves. For example, gasoline-electric hybrid vehicles sold in the US, operated on gasoline (not E85), are rated at 56-60 mpg city, but typically achieve only 31-33 mpg for the average driver that ventures outside the city.

In the United Kingdom, the Vehicle Certification Agency (VCA) has initiated a similar fuel economy rating system in accordance with European Community Directive 93/116/EC. The ratings are based on an urban and extra-urban driving cycle. The urban cycle is a cold start followed by "a series of accelerations, steady speeds, decelerations and idling. Maximum speed is 31mph (50km/h), average speed 12mph (19km/h) and the distance covered is 2.5 miles (4km)." The extra-urban cycle is conducted immediately following the urban cycle and consists of roughly half steady-speed driving and the remainder accelerations, decelerations, and some idling. Maximum speed is 75mph (120km/h), average speed is 39mph (63 km/h) and the distance covered is 4.3miles (7km).

The raw averages for all 2005 vehicles rated in the UK are: Urban cycle, 11.3, extra-urban 6.4 (L/100 km). This converts to 20.9 and 36.5 mpg, respectively, in U.S. measurements.


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


  • Your vehicle's weight (especially in city driving) and frontal area (especially in highway driving) are the two largest contributing factors.
  • Your engine is a major contributing factor.

The most efficient mass-production engines are petrol engines under a litre displacement, turbo-diesels under 1.4 litres, standard diesels and electric hybrids. Out of these, turbo-diesels tend to be the most efficient in terms of power per unit displacement. However, if acceleration rates were not to concern you, a standard diesel engine is the most efficient per litre displacement.

  • 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. Temporarily shifting to neutral on a sufficiently lengthy downhill grade will dramatically increase mileage for carburetor cars, while cars with fuel injection - or carburetor cars with a fuel cut-off solenoid - will benefit more from the fuel cutoff when the car is left in gear.
  • 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. However, driving at low speeds with the air conditioner OFF and the windows open can improve 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 5-25%. 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 high speeds. Gasoline powered cars operate with maximum efficiency in the highest gear at the lowest speed in that highest gear without engine lugging. This effect is largely due to aerodynamic drag. In highway driving over 80-90 km/h (50-55 mph) (depending on the car's aerodynamics; low for small, aerodynamic cars and high for SUV, trucks), the aerodynamic drag will rise sharply and so rises the fuel consumption.

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