1 distance measured in miles [syn: milage]
2 the ratio of the number of miles traveled to the number of gallons of gasoline burned [syn: fuel consumption rate, gasoline mileage, gas mileage]
3 a travel allowance at a given rate per mile traveled
Fuel economy in automobiles is the amount of fuel required to move the automobile over a given distance. While the fuel efficiency of petroleum engines has improved markedly in recent decades, this does not necessarily translate into fuel economy of cars, as people in developed countries tend to buy bigger and heavier cars.
Units of measureThe two most common ways to measure automobile fuel economy are:
- 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.
- The distance travelled per unit of fuel used; most commonly kilometres per litre (km/L) or miles per gallon (usually referred to as mpg). 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 2.3 kg of carbon dioxide for each litre of petrol consumed (19 lb/gal). A typical diesel engine produces 2.8 kg/L (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.
Diminishing returnBecause miles per gallon is the inverse of actual fuel consumption (gallons per mile), each additional mile per gallon represents less saved fuel. For example, switching from a 10 mpg vehicle to a 15 mpg vehicle saves three times as much gas as switching from a 50 mpg vehicle to a 100 mpg vehicle.
A vehicle getting 10 mpg would consume 0.1 gallons per mile (1/mpg). Reducing fuel consumption by increments of 0.01 gal/mile results in the following measurements:
- 11.1 mpg (0.09 gal/mile)
- 12.5 mpg (0.08 gal/mile)
- 14.3 mpg (0.07 gal/mile)
- 16.7 mpg (0.06 gal/mile)
- 20 mpg (0.05 gal/mile)
- 25 mpg (0.04 gal/mile)
- 33.3 mpg (0.03 gal/mile)
- 50 mpg (0.02 gal/mile)
- 100 mpg (0.01 gal/mile)
Fuel economy statisticsThe 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 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 3401 km/l, or more than 8000 mpg .
Both such vehicles are extremes, and most people drive ordinary cars that typically average 15 to 40 miles per US gallon ( 19 to 50 miles per imperial gallon) or (5.6 to 15 L per 100 km). However, due to environmental concerns caused by CO2 emissions, new EU regulations are being introduced to reduce the average emissions, of cars sold beginning in 2012, to 130 g/km of CO2, equivalent to 4.5 L per 100 km (52 mpg US, 63 MPG imperial) for a diesel-fueled car, and 5.0 L per 100 km (47 mpg US, 56 MPG imperial) for a gasoline (petrol)-fueled car. EU fuel economy testing is done on a rolling road with two segments, ECE15 and EUDC, which correspond to city and highway driving, respectively. The city driving cycle simulates a 4.052 km (2.5 mile) urban trip at an average speed of 18.7 km/h (11.6 mph) and at a maximum speed of 50 km/h (31 mph), while the highway cycle lasts 400 seconds (6 minutes 40 seconds) at an average speed 62.6 km/h (39 mph) and a top speed of 120 km/h (74.6 mph).
It should be borne in mind that the average consumption across the fleet is not immediately directly affected by the new vehicle fuel economy, for example Australia's car fleet average in 2004 was 11.5 l/100 km (20.5 mpg), compared with the average new car consumption in the same year of 25.3 mpUSg.
Physics backgroundThe power to overcome air resistance increases roughly with the cube of the speed, and thus the energy required per unit distance is roughly proportional to the square of speed. Because air resistance increases so rapidly with speed, above about 30 mph (48 km/h), it becomes a dominant limiting factor. Driving at 45 rather than 65 mph (72 rather than 105 km/h), results in about one-third the power to overcome wind resistance, or about one half the energy per unit distance, and much greater fuel economy can be achieved. Increasing speed to 90 mph (145 km/h) from 65 mph (105 km/h) increases the power requirement by 2.6 times, the energy by 1.9 times, and drastically decreases fuel economy. In practice, rather than doubling or halving the fuel economy, the difference is actually closer to 40-50%, because engine efficiency varies greatly with the torque/speed operating point. Rolling resistance, which is broadly proportional to speed, is also a factor particularly at lower speeds.
There were complaints when the speed limit was lowered from 65 mph to 55 mph that it could lower, instead of increase fuel economy, and in fact the 1997 Toyota Celica gets 1 mpg better mileage at 65 than it does at 55 (43.5 vs 42.5), although almost 5 mpg better at 60 than at 65 (48.4 vs 43.5). Other vehicles tested had from 1.4 to 20.2% better mileage at 55 mph vs. 65 mph.
U.S. government regulations
U.S. Energy Tax ActThe Energy Tax Act of 1978 in the U.S. established a gas guzzler tax on the sale of new model year vehicles whose fuel economy fails to meet certain statutory levels. The tax applies only to cars (not trucks) and is collected by the IRS. Its purpose is to discourage the production and purchase of fuel-inefficient vehicles. The tax was phased in over ten years with rates increasing over time. It applies only to manufacturers and importers of vehicles, although presumably some or all of the tax is passed along to automobile consumers in the form of higher prices. Only new vehicles are subject to the tax, so no tax is imposed on used car sales. The tax is graduated to apply a higher tax rate for less-fuel-efficient vehicles. To determine the tax rate, manufacturers test all the vehicles at their laboratories for fuel economy. The U.S. Environmental Protection Agency confirms a portion of those tests at an EPA lab.
EPA testing procedure through 2007Two separate fuel economy tests simulate city driving and highway driving: the city driving program consists of starting with a cold engine and making 23 stops over a period of 31 minutes for an average speed of 20 mph (32 km/h) and with a top speed of 56 mph (90 km/h); the highway program uses a warmed-up engine and makes no stops, averaging 48 mph (77 km/h) with a top speed of 60 mph (97 km/h) over a 10 mile (16 km) distance. The measurements are then adjusted downward by 10% (city) and 22% (highway) to more accurately reflect real-world results. A weight average of city (55%) and highway (45%) fuel economies is used to determine the tax.
In some cases, this tax may only apply to certain variants of a given model - for example, the 2004–2006 Pontiac GTO did incur the tax when ordered with the four-speed automatic transmission, but did not incur the tax when ordered with the six-speed manual transmission.
Because EPA figures are almost always higher than real-world mileage, EPA has modified the method starting with 2008.
New 2008– EPA testing procedureAs a means of reflecting real world fuel economy more accurately, the EPA adds three new tests that will combine with the current city and highway cycles to determine fuel economy of new vehicles, beginning with the 2008 model year. A high speed/quick acceleration loops lasts 10 minutes, covers , averages and reaches a top speed of . Four stops are included, and brisk acceleration maximizes at a rate of per second. The engine begins warm and air conditioning is not used. Ambient temperature varies between 68 to .
The air conditioning test raises ambient temperatures to , and the vehicle's climate control system is put to use. Lasting 9.9 minutes, the loop averages and maximizes at a rate of . Five stops are included, idling occurs 19 percent of the time and acceleration of 5.1 mph/sec is achieved. Engine temperatures begin warm. Lastly, a cold temperature cycle uses the same parameters as the current city loop, except that ambient temperature is set to .
CAFE standardsThe Corporate Average Fuel Economy (CAFE) regulations in the United States, first enacted by Congress in 1975, are federal regulations intended to improve the average fuel economy of cars and light trucks (trucks, vans and sport utility vehicles) sold in the US in the wake of the 1973 Arab Oil Embargo. Historically, it is the sales-weighted average fuel economy of a manufacturer's fleet of current model year passenger cars or light trucks, manufactured for sale in the United States. Under Truck CAFE standards 2008–2011 this changes to a "footprint" model where larger trucks are allowed to consume more fuel. The standards are limited to vehicles under a certain weight, but those weight classes will be expanding in 2011 if current law (as of April 2006) holds.
State regulationsThe states are pre-empted by federal law, and are not allowed to make fuel efficiency standards. However, California has a special dispensation from the Clean Air Act to make emissions standards (which other states may adopt instead of the federal standards). The California Air Resources Board is implementing some legislation which limits greenhouse gas emissions. A legal dispute has emerged over whether this is effectually a fuel efficiency standard.
European standardsIn the European Union advertising has to show CO2-emission and fuel consumption data in a clear way as described in the UK Statutory Instrument 2004 No 1661. The industry usually places this information in footnotes along with price and other details.
Energy considerationsIdeally, a car traveling at a constant velocity and constant grade in a vacuum with frictionless wheels could travel at any speed without consuming any energy beyond what is needed to get the car up to speed. With ideal regenerative braking, this energy could be completely recovered. In real-world conditions, energy is lost in a number of ways:
- Engine efficiency, which varies with engine type, the mass of the automobile and its load, and engine speed (usually measured in RPM).
- Aerodynamic drag force, which increases roughly by the square of the car's speed, but note that Aerodynamic drag power, increases roughly by the cube of the car's speed.
- Rolling friction.
- Braking, although regenerative braking captures some of the energy that would otherwise be lost.
- Losses in the transmission. (Manual transmissions can be up to 94% efficient whereas older automatic transmissions may be as low as 70% efficient. Automatically controlled shifting of gearboxes that have the same internals as manual boxes will give the same efficiency as a pure manual gearbox plus the bonus of added intelligence selecting optimal shifting points
- Air conditioning. Parasitic losses due to the necessary power required for the engine to turn the compressor additionally decrease fuel mileage, though only when in use.
- Electrical systems. Headlights, media systems, speakers, and other electronics can also increase fuel consumption, as the energy to power these devices causes increased load on the alternator.
Fuel economy-boosting technologies (see also Low-energy vehicle)
- Reducing vehicle weight by using materials such as aluminum, fiberglass, plastic, high-strength steel and carbon fiber instead of steel and iron
- Designing the exterior of the vehicle to reduce aerodynamic drag
- Using lower-viscosity lubricants (engine oil, transmission fluid, axle fluid)
- Incorporating Locking torque converters in automatic transmissions to reduce slip and power losses in the converter
- Augmenting a downsized engine with an electric drive system and battery (hybrid vehicles) hybrid electric vehicle
- Automatically shutting off engine when vehicle is stopped (mild hybrid)
- Recapturing wasted energy while braking (regenerative braking)
- Optimizing other engine combustion strategies:
Many aftermarket consumer products exist which are purported to increase fuel economy; many of these claims have been discredited. In the United States, the Environmental Protection Agency maintains a list of devices that have been tested by independent laboratories and makes the test results available to the public.
Fuel economy data reliabilityThe mandatory publication of the fuel consumption by the manufacturer led some to use dubious practices to reach better values in the past. If the test is on a test stand, the vehicle may detect open doors and adapt the engine control. Also when driven according to the test regime, the parameters may adapt automatically. Test laboratories use a "golden car" that is tested in each one to check that each lab produces the same set of measurements for a given drive cycle.
Correctly aligning the vehicle wheels is something that should be normal practice for the vehicle users. Tire pressures and lubricants have to be as recommended by the manufacturer (Higher tire pressures are required on a particular dyno type, but this is to compensate for the different rolling resistance of the dyno, not to produce an unrealistic load on the vehicle). Normally the quoted figures a manufacturer publishes have to be proved by the relevant authority witnessing vehicle/engine tests. A lot of Governments independently test emissions from customer vehicles, and as a final measure can force a recall of all of a particular type of vehicle if the customer vehicles do not fulfil manufacturers' claims within reasonable limits. The expense and bad publicity from such a recall means manufacturers should be very cautious not to publish unrealistic figures. The US Federal government retests 10-15% of models), to make sure that the manufacturer's tests are accurate.
Fuel economy maximizing behaviorsGovernments, various environmentalist organizations, and companies like Toyota and Shell Oil Company have historically urged drivers to maintain adequate air pressure in tires and careful acceleration/deceleration habits.
Fuel economy as part of quality management regimesEnvironmental management systems EMAS as well as good fleet management do include record keeping of the fuel consumption of the fleet. Quality management on top of this uses those figures to steer the measures acting on the fleets. You may check whether procurement, driving, and maintenance in total have contributed to changes in the fleets overall consumption.
- [http://www.epa.gov:80/fueleconomy/label.htm U.S. Fuel Economy Label] (EPA).
- US EPA Green Vehicle Guide.
- European Community Directive 93/116/EC — European Commission Directive 93/116/EC of 17.12.1993 adapting to technical progress Council Directive 80/1268/EEC relating to the fuel consumption of motor vehicles
- Online MPG Calculator.
mileage in German: Kraftstoffverbrauch
mileage in Spanish: Ahorro de combustible in automóviles
mileage in Dutch: Brandstofverbruik
mileage in Japanese: 燃費
mileage in Russian: Расход топлива автомобилий
mileage in Swedish: Bränsleförbrukning
account, aesthetic distance, allowance, assessment, bill, blackmail, blood money, clearance, compass, deep space, depths of space, distance, divergence, emolument, extension, extent, farness, fee, footing, hush money, infinity, initiation fee, leeway, length, lengthiness, light-years, linear measures, long time, longitude, longness, margin, measure, overall length, parsecs, perpetuity, perspective, piece, range, reach, reckoning, remoteness, retainer, retaining fee, scot, separation, space, span, stipend, stretch, stride, tribute, way, ways, yardage