Fuel economy:

A 1916 experiment in creating a fuel-economic automobile in the United States. The vehicle weighed only 135 pounds and was an adaptation of a small gasoline engine originally designed to power a bicycle.^[1]

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, if people buy bigger and heavier cars.

Units of measure

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)^[2]^[3] 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.^[4]

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 kilometers per liter. 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. ^[5].

Both such vehicles are extremes, and most people drive ordinary cars that typically average 15 to 40 miles per U.S. 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.^[6] 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 4052 m (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).^[7]

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-U.S.),^[8] compared with the average new car consumption in the same year of 25.3 mpg-U.S.^[9]

New Zealand fuel economy ratings

United Kingdom fuel economy ratings

United States EPA fuel economy ratings

Theoretical Physics

The 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 decreases fuel economy.

The power needed to overcome the rolling resistance, which is broadly proportional to speed, is also a factor particularly at lower speeds. At very low speeds the dominant losses are internal friction. A hybrid can get better gas mileage in city driving than on the highway because the gas engine shuts off when it is not needed to charge the battery, and has little to no consumption at stops. In addition regenerative braking puts energy back into the battery.

Speed and Fuel Economy Studies

The most recent study of fuel economy at various speeds is from 1997 and primarily tested (then) 3- or 4-year older vehicles. That study showed that some vehicles achieve better mileage at 65 than at 45-mph, although not their best economy, such as the 1994 Oldsmobile Cutlass, which has its best economy at 55 mph (29.1 mpg), and gets 2 mpg better economy at 65 than at 45 (25 vs 23 mpg). All cars have decreasing fuel economy beyond 65 mph (105 km/h), with wind resistance the dominant factor, and save a maximum of 25% by slowing from 70 mph to 55 mph.^[10]

There were complaints when the U.S. National 55-mph speed limit was mandated that it could lower, instead of increase fuel economy. The newest vehicle tested in 1997, a 1997 Toyota Celica, got 1 mpg better fuel-efficiency 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), and its best economy (52.6 mpg) is at only 25 mph. Other vehicles tested had from 1.4 to 20.2% better fuel-efficiency at 55 mph vs. 65 mph, and their best economy was reached at speeds of 25 to 55 mph.^[11]

Fuel economy standards

Australia

Beginning in October, 2008, all new cars will need to be sold with a sticker on the windscreen showing the fuel economy and the CO₂ emissions.^[13] Australia uses a star rating system, from one to five stars, but it combines greenhouse gases with pollution, rating each from 0 to 10 with ten being best. To get 5 stars a combined score of 16 or better is needed, so a car with a 10 for economy (greenhouse) and a 6 for emission or 6 for economy and 10 for emission, or anything in between would get the highest 5 star rating.^[14] The lowest rated car is the Ssangyong Korrando Ssangyong with automatic transmission, with one star, while the highest rated was the Toyota Prius hybrid. The Fiat 500, Fiat Punto and Fiat Ritmo as well as the Citroen C3 also received 5 stars.^[15] The greenhouse rating depends on the fuel economy and the type of fuel used. A greenhouse rating of 10 requires 60 or less grams of CO₂ per km, while a rating of zero is more than 440 g/km CO₂. The highest greenhouse rating of any 2009 car listed is the Toyota Prius, with 106 g/km CO₂ and 4.4 L/100 km (53 mpg–U.S. / 64 mpg–imp). Several other cars also received the same rating of 8.5 for greenhouse. The lowest rated was the Ferrari 575 at 499 g/km CO₂ and 21.8 L/100 km (11 mpg–U.S. / 13 mpg–imp). The Bentley also received a zero rating, at 465 g/km CO₂. The best fuel economy of any year is the 2004-2005 Honda Insight, at 3.4 L/100 km (69 mpg–U.S. / 83 mpg–imp).

Europe

In 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.^[16] Since September 2005 a color-coded "Green Rating" sticker has been available in the UK which rates fuel economy by CO₂ emissions: A: <= 100 g/km, B: 100 - 120, C: 121 - 150, D: 151 - 165, E: 166 - 185, F: 186 - 225, and G: 226+. Depending on the type of fuel used, for gasoline A corresponds to about 4.1 L/100 km (57 mpg–U.S. / 69 mpg–imp) and G about 9.5 L/100 km (25 mpg–U.S. / 30 mpg–imp).^[17] Ireland has a very similar label, but the ranges are slightly different, with A: <= 120 g/km, B: 121 - 140, C: 141 - 155, D: 156 - 170, E: 171 - 190, F: 191 - 225, and G: 226+.^[18]

New Zealand

Starting on 7 April 2008 all cars of up to 3.5 tonnes GVW sold other than private sale need to have a fuel economy sticker applied (if available) which shows the rating from one half star to six stars with the best economy cars having the most stars and the worst gas guzzlers the least, along with the fuel economy in L/100 km and the estimated annual fuel cost for driving 14,000 km. The stickers must also appear on vehicles to be leased for more than 4 months. All new cars currently rated range from 6.9 L/100 km (34 mpg–U.S. / 41 mpg–imp) to 3.8 L/100 km (62 mpg–U.S. / 74 mpg–imp) and received respectively from 4 1/2 to 5 1/2 stars.^[19]

United States

U.S. Energy Tax Act

The Energy Tax Act of 1978^[20] 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 2007

Two 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.^[21]

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 had almost always indicated better efficiency than real-world fuel-efficiency, the EPA has modified the method starting with 2008. Updated estimates are available for vehicles back to the 1985 model year.^[22]

EPA testing procedure: 2008 and beyond

As a means of reflecting real world fuel economy more accurately, the EPA adds three new tests^[23] 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 8 miles (13 km), averages 48 mph (77 km/h) and reaches a top speed of 80 mph (130 km/h). Four stops are included, and brisk acceleration maximizes at a rate of 8.46 mph (13.62 km/h) per second. The engine begins warm and air conditioning is not used. Ambient temperature varies between 68 to 86 °F (30 °C).

The air conditioning test raises ambient temperatures to 95 °F (35 °C), and the vehicle's climate control system is put to use. Lasting 9.9 minutes, the 3.6-mile (5.8 km) loop averages 22 mph (35 km/h) and maximizes at a rate of 54.8 mph (88.2 km/h). 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 20 °F (−7 °C).

EPA tests for fuel economy do not include electrical load tests beyond climate control which may account for some of the discrepancy between EPA and real world fuel-efficiency. A 200-W electrical load can produce a 0.4 km/L reduction in efficiency on the FTP 75 cycle test.^[24]

CAFE standards

The Corporate Average Fuel Economy (CAFE) regulations in the United States, first enacted by Congress in 1975,^[25] 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 regulations

The 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.

Energy considerations

Ideally, a car traveling at a constant velocity on level ground 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:

Fuel-efficiency decreases from electrical loads are most pronounced at lower speeds because most electrical loads are constant while engine load increases with speed. So at a lower speed a higher proportion of engine horsepower is used by electrical loads. Hybrid cars see the greatest effect on fuel-efficiency from electrical loads because of this proportional effect.

Fuel economy-boosting technologies (see also Low-energy vehicle)

Future Technologies

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.^[27]

Fuel economy data reliability

The 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.^[28]

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^[29]), to make sure that the manufacturer's tests are accurate.

Fuel economy maximizing behaviors

Governments, 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 regimes

Environmental 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.^[30]

Fuel economy

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