Miles per gallon gasoline equivalent

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Monroney label showing the EPA's fuel economy equivalent ratings for the 2011 Chevrolet Volt. The rating for all-electric mode (left) is expressed in miles per gallon gasoline equivalent.

Miles per gallon gasoline equivalent (MPGe or MPGge) is a measure of the average distance traveled per unit of energy consumed. MPGe is used by the U.S. Environmental Protection Agency (EPA) to compare energy consumption of alternative fuel vehicles, plug-in electric vehicles and other advanced technology vehicles with the fuel economy of conventional internal combustion vehicles expressed as miles per US gallon.[1][2]

The MPGe metric was introduced in November 2010 by EPA in the Monroney label of the Nissan Leaf electric car and the Chevrolet Volt plug-in hybrid. The ratings are based on EPA's formula, in which 33.7 kilowatt hours of electricity is equivalent to one gallon of gasoline, and the energy consumption of each vehicle during EPA's five standard drive cycle tests simulating varying driving conditions.[3][4] All new cars and light-duty trucks sold in the U.S. are required to have this label showing the EPA's estimate of fuel economy of the vehicle.[2]

In a joint ruling issued in May 2011 the National Highway Traffic Safety Administration (NHTSA) and EPA established the new requirements for a fuel economy and environment label that is mandatory for all new passenger cars and trucks starting with model year 2013. This ruling uses miles per gallon gasoline equivalent for all fuel and advanced technology vehicles available in the U.S. market including plug-in hybrids, electric vehicles, flexible-fuel vehicles, hydrogen fuel cell vehicle, natural gas vehicles, diesel-powered vehicles, and gasoline-powered vehicles.[5][6] In addition to being displayed on new vehicles, fuel economy ratings are used by the U.S. Department of Energy (DOE) to publish the annual Fuel Economy Guide; the U.S. Department of Transportation (DOT) to administer the Corporate Average Fuel Economy (CAFE) program; and the Internal Revenue Service (IRS) to collect gas guzzler taxes.[2]

Fuel economy estimates for window stickers and CAFE standard compliance are different. The EPA MPGe rating shown in the Monroney label is based on the consumption of the on-board energy content stored in the fuel tank or in the vehicle's battery, or any other energy source, and only represents the tank-to-wheel energy consumption. CAFE estimates are based on a well-to-wheel basis and in the case of liquid fuels and electric drive vehicles also account for the energy consumed upstream to produce the fuel or electricity and deliver it to the vehicle. Fuel economy for CAFE purposes include an incentive adjustment for alternative fuel vehicles and plug-in electric vehicles which results in higher MPGe than those estimated for window stickers.[7][8]

Background[edit]

1988: Alternative Motor Fuels Act[edit]

The Alternative Motor Fuels Act (AMFA) enacted in 1988 provides Corporate Average Fuel Economy (CAFE) incentives for the manufacture of vehicles that use ethanol, methanol, or natural gas fuels, either powered exclusively on these alternative fuels or in conjunction with gasoline or diesel fuel, such flexible-fuel vehicles. In order to provide incentives for the widespread use of these fuels and to promote the production of alternative fuel vehicles, AMFA allows manufacturers producing alternative fuel vehicles to gain CAFE credits by manufacturing these vehicles, which allows them to raise their overall fleet fuel economy levels to comply with the CAFE standards until the established cap level.[9][10]

Beginning in 1993, manufacturers of qualified alternative fuel vehicles can benefit for their CAFE estimation, by computing the weighted average of the fuel economy of the produced alternative fuel vehicles by dividing the alcohol fuel economy by a factor of 0.15. As an example, a dedicated alternative fuel vehicle that would achieve 15 mpg fuel economy while operating on alcohol would have a CAFE calculated as follows:[10]

FE = (1/0.15)(15) = 100 miles per gallon

For alternative dual-fuel vehicles, an assumption is made that the vehicles would operate 50% of the time on the alternative fuel and 50% of the time on conventional fuel, resulting in a fuel economy that is based on a harmonic average of alternative fuel and conventional fuel. For example, for an alternative dual-fuel model that achieves 15 miles per gallon operating on an alcohol fuel and 25 mpg on the conventional fuel, the resulting CAFE would be:[10]

FE = 1 / [(0.5/25) + (0.5/100)] = 40 miles per gallon

Calculation of fuel economy for natural gas vehicles is similar. For the purposes of this calculation, the fuel economy is equal to the weighted average of the fuel economy while operating on natural gas and while operating on either gasoline or diesel fuel. AMFA specifies the energy equivalency of 100 cubic feet of natural gas to be equal to 0.823 gallons of gasoline, with the gallon equivalency of natural gas to be considered to have a fuel content, similar to that for alcohol fuels, equal to 0.15 gallons of fuel. For example, under this conversion and gallon equivalency, a dedicated natural gas vehicle that achieves 25 miles per 100 cubic feet of natural gas would have a CAFE value as follows:[10]

FE = (25/100) x (100/0.823)(1/0.15) = 203 miles per gallon

The Energy Policy Act of 1992 expanded the definition of alternative fuel to include liquefied petroleum gas, hydrogen, liquid fuels derived from coal and biological materials, electricity and any other fuel that the Secretary of Transportation determines to be substantially non-petroleum based and has environmental and energy security benefits. Beginning in 1993, manufacturers of these other alternative fuel automobiles that meet the qualifying requirements can also benefit for special treatment in the calculation of their CAFE.[10]

1994: Gasoline gallon equivalent[edit]

In 1994 the U.S. National Institute of Standards and Technology (NIST) introduced gasoline gallon equivalent (GGE) as a metric for fuel economy for natural gas vehicles. NIST defined a gasoline gallon equivalent (GGE) as 5.660 pounds of natural gas, and gasoline liter equivalent (GLE) as 0.678 kilograms of natural gas.[11]

2000: Petroleum-equivalent fuel economy[edit]

Energy efficiency for selected electric cars
leased in California between 1996–2003
Vehicle Model
year
Type of
battery
Energy use
(Wh/mi)
GM EV1[12] 1997 Lead acid 164
GM EV1[13] 1999 NiMH 179
Toyota RAV4 EV[14] 1996 Lead acid 235
Toyota RAV4 EV[15] 2000 NiMH 400
Ford Ranger EV[16] 1998 Lead acid 337
Chevrolet S-10 EV[17] 1997 Lead acid 292

During the late 1990s and early 2000s several electric cars were produced in limited quantities as a result of the California Air Resources Board (CARB) mandate for more fuel-efficient zero-emissions vehicles. Popular models available in California included the General Motors EV1 and the Toyota RAV4 EV.[18][19] The US DoE and EPA rating for on board energy efficiency for these electric vehicles was expressed as kilowatt-hour/mile (KWh/mi), the most commonly known metric in science and engineering for measuring energy consumption, and used as the billing unit for energy delivered to consumers by electric utilities.[20]

In order to address the Corporate Average Fuel Economy (CAFE) regulations mandated by the US Congress in 1975, the U.S. Department of Energy established in July 2000 a methodology for calculating the petroleum-equivalent fuel economy of electric vehicles on a well-to-wheel basis. The methodology considers the upstream efficiency of the processes involved in the two fuel cycles, including efficiency factors for petroleum refining and distribution, as well as the national average efficiency for electricity generation and transmission.[8] The formula also includes a fuel efficiency incentive factor of 1/0.15 to benefit electric vehicles. This reward factor is intended provide an incentive for vehicle manufactures to produce and sell electric vehicles, as a higher equivalent fuel economy for EVs improves the carmaker overall fleet fuel economy levels in complying with the CAFE standards, and Congress anticipated that such an incentive would help accelerate the commercialization of electric vehicles. The incentive factor chosen by DoE for EVs is the same 1/0.15 factor already applied in the regulatory treatment of other types of alternative fuel vehicles.[8] When all factors are considered in DoE's formula, the energy efficiency or equivalent fuel economy of electric vehicles increases from 33,705 Wh/gallon (plug-to-wheel) to 82,049 Wh/gallon (well-to-wheel).[20]

2007: X Prize[edit]

In April 2007, as part of Draft Competition Guidelines released at the New York Auto Show, MPGe was announced as the main merit metric for the Progressive Insurance Automotive X Prize, a competition developed by the X Prize Foundation for super-efficient vehicles that can achieve at least 100 MPGe.[21] In February 2009, Consumer Reports announced that, as part of a partnership with the X Prize Foundation, they planned to report MPGe as one of several measures that will help consumers understand and compare vehicle efficiency for alternative fuel vehicles.[22]

2010–2011: Miles per gallon equivalent[edit]

Old Monroney label for electric cars showing in prominent larger font the fuel economy rating in KW-hr/100 miles for the 2009 Mini E.
New Monroney label for electric cars showing in prominent larger font the fuel economy rating in miles per gallon gasoline equivalent for the 2011 Nissan Leaf. The rating in KW-hr/100 miles is shown below MPG-e in smaller font.

As required by the 2007 Energy Independence and Security Act (EISA), with the introduction of advanced-technology vehicles in the U.S. new information should be incorporated in the Monroney label of new cars and light-duty trucks sold in the country, such as ratings on fuel economy, greenhouse gas emissions, and other air pollutants. The U.S. Environmental Protection Agency and the National Highway Traffic Safety Administration (NHTSA) have conducted a series of studies to determine the best way to redesign this label to provide consumers with simple energy and environmental comparisons across all vehicles types, including battery electric vehicles (BEV), plug-in hybrid electric vehicles (PHEV), and conventional internal combustion engine vehicles powered by gasoline and diesel, in order to help consumers choose more efficient and environmentally friendly vehicles. These changes were proposed to be introduced in new vehicles beginning with model year 2012.[2][23]

The EPA rating for on board energy efficiency for electric vehicles before 2010 was expressed as kilowatt-hour per 100 miles (kW-hrs/100 mi).[20][24] The window sticker of the 2009 Mini E showed an energy consumption of 33 kW-hrs/100 mi for city driving and 36 kW-hrs/100 mi on the highway, technically equivalent to 102 mpg city and 94 mpg highway.[24] The 2009 Tesla Roadster was rated 32 kW-hrs/100 mi in city and 33 kW-hrs/100 mi on the highway, equivalent to 105 mpg city and 102 mpg highway.[25][26]

As part of the research and redesign process, EPA conducted focus groups where participants were presented with several options to express the consumption of electricity for plug-in electric vehicles. The research showed that participants did not understand the concept of a kilowatt hour as a measure of electric energy use despite the use of this unit in their monthly electric bills. Instead, participants favored a miles per gallon equivalent, MPGe, as the metric to compare with the familiar miles per gallon used for gasoline vehicles. The research also concluded that the kW-hrs per 100 miles metric was more confusing to focus group participants compared to a miles per kW-hr. Based on these results, EPA decided to use the following fuel economy and fuel consumption metrics on the redesigned labels: MPG (city and highway, and combined); MPGe (city and highway, and combined); Gallons per 100 miles; kW-hrs per 100 miles.[23]

The proposed design and final content for two options of the new sticker label that would be introduced in 2013 model year cars and trucks were consulted for 60 days with the public in 2010, and both include miles per gallon equivalent and kW-hrs per 100 miles as the fuel economy metrics for plug-in cars, but in one option MPGe and annual electricity cost are the two most prominent metrics.[27][28] In November 2010, EPA introduced MPGe as comparison metric on its new sticker for fuel economy for the Nissan Leaf and the Chevrolet Volt.[3][4]

Typical label for hydrogen fuel cell vehicles expressed in MPGe, mandatory starting with 2013 model year.

In May 2011, the National Highway Traffic Safety Administration (NHTSA) and EPA issued a joint final rule establishing new requirements for a fuel economy and environment label that is mandatory for all new passenger cars and trucks starting with model year 2013. The ruling includes new labels for alternative fuel and alternative propulsion vehicles available in the US market, such as plug-in hybrids, electric vehicles, flexible-fuel vehicles, hydrogen fuel cell vehicle, and natural gas vehicles.[5][6] The common fuel economy metric adopted to allow the comparison of alternative fuel and advanced technology vehicles with conventional internal combustion engine vehicles is miles per gallon of gasoline equivalent (MPGe). A gallon of gasoline equivalent means the number of kilowatt-hours of electricity, cubic feet of compressed natural gas (CNG), or kilograms of hydrogen that is equal to the energy in a gallon of gasoline.[5]

The new labels also show for the first time an estimate of how much fuel or electricity it takes to drive 100 miles (160 km), introducing to U.S. consumers with fuel consumption per distance traveled, the metric commonly used in many other countries. EPA explained that the objective is to avoid the traditional miles per gallon metric that can be potentially misleading when consumers compare fuel economy improvements, and known as the "MPG illusion."[5]

Description[edit]

The miles per gallon gasoline equivalent is based on the energy content of gasoline. The energy obtainable from burning one US gallon is 115,000 BTU (34 kWh). Thus one mile per gallon gasoline equivalent is equal to 115,000 BTU per mile.[29] For alternative fuels, energy required to manufacture the fuel may also be considered. For electrical power, the energy of any fuels used to generate the electricity and the transmission efficiency must be considered.

To convert the mile per gallon rating into other units of distance per unit energy used, the mile per gallon value can be multiplied by one of the following factors to obtain other units:

1 MPGe = 8.55 miles/ million BTU
≈ 0.0292 miles/kW·h
≈ 0.0470 km/kW·h
≈ 0.013 km/MJ

Conversion to MPGe[edit]

MPGe is determined by converting the vehicle consumption per unit distance, as determined through computer modeling or completion of an actual driving cycle, from its native units into a gasoline energy equivalent. Examples of native units include W·h for electric vehicles, kg-H2 for hydrogen vehicles, gallons for biodiesel or liquefied natural gas vehicles, cubic feet for compressed natural gas vehicles, and pounds for propane or Liquefied petroleum gas vehicles. Special cases for specific alternative fuels are discussed below, but a general formula for MPGe is:

 MPGe = \frac{total~miles~driven}{\left [ \frac{total~energy~of~all~fuels~consumed}{energy~of~one~gallon~of~gasoline} \right ]} = \frac{(total\ miles\ driven) \times (energy\ of\ one\ gallon\ of\ gasoline)}  {total~energy~of~all~fuels~consumed}

For EPA, this considers the tank-to-wheel for liquids and wall-to-wheel energy consumption for electricity, i.e. it measures the energy for which the owner usually pays. For EVs the energy cost includes the conversions from AC to charge the battery.[30] The EPA MPGe ratings displayed in window stickers do not account for the energy consumption upstream, which includes the energy or fuel required to generate the electricity or to extract and produce the liquid fuel; the energy losses due to power transmission; or the energy consumed for the transportation of the fuel from the well to the station.[8][31]

Basic values for the energy content of various fuels are given by the defaults used in the Department of Energy GREET (Greenhouse gases, Regulated Emissions, and Energy used in Transportation) model,[32] as follows:

Note: 1 KWH is equivalent to 3,412 BTU

Fuel Unit Btu/Unit KWH/Unit
gasoline gallon 116,090 34.02
diesel gallon 129,488 37.95
biodiesel gallon 119,550 35.04
ethanol gallon 76,330 22.37
E85 gallon 82,000 24.03
CNG 100 SCF 98,300 28.81
H2-Gas 100 SCF 28,900 8.47
H2-Liq gallon 30,500 8.94
LPG gallon 84,950 24.9
methanol gallon 57,250 16.78

The energy content of a particular fuel can vary somewhat given its specific chemistry and production method. For example, in the new efficiency ratings that have been developed by the United States Environmental Protection Agency (EPA) for battery electric vehicles (BEVs) and plug-in hybrid electric vehicles (PHEVs) – see below – the energy content of a gallon of gasoline is assumed to be 114,984 BTUs.[citation needed]

Electric and plug-in hybrid electric vehicles[edit]

Monroney label showing the EPA's fuel economy equivalent ratings for the 2011 Smart ED electric car.

Between 2008 and 2010 several major automakers began commercializing battery electric vehicles (BEVs), which are powered exclusively on electricity, and plug-in hybrid electric vehicles (PHEVs), which use electricity together with a liquid fuel stored in an on-board fuel tank, usually gasoline, but it might be also powered by diesel, ethanol, or flex-fuel engines.

For battery electric vehicles, the U.S. Environmental Protection Agencys formula to calculate the battery-to wheel MPGe is based on energy standards established by the U.S. Department of Energy in 2000:[1][7][8]

 MPGe = \frac { E_G} {E_M*E_E} = \frac{ 33,705 } {E_M}

where

 MPGe is expressed as miles per gallon gasoline equivalent (as shown in the Monroney label)
  E_G = energy content per gallon of gasoline = 115,000 BTUs/gallon, as set by U.S. DoE and reported by the Alternative Fuel Data Center.[8]
  E_M = wall-to-wheel electrical energy consumed per mile (Wh/mi) as measured through EPA's five standard drive cycle tests for electric cars and SAE test procedures[7][30]
  E_E = energy per KWatt-hour of electricity (BTU/Wh) = 3.412[8]

The formula employed by the EPA for calculating their rated MPGe does not account for any fuel or energy consumed upstream such as the generation and transmission of electrical power, or well-to-wheel life cycle, as EPA's comparison with internal combustion vehicles is made on a tank-to-wheel versus battery-to wheel basis.

The California Air Resources Board uses a different dynamometer testing than EPA, and considers reformulated gasoline sold in that state. For CARB estimates the formula becomes:[7]

 MPGe = \frac { E_G} {E_M*E_E} = \frac{ 32,600 } {E_M}

The new SAE J1711 standard for measuring the exhaust emissions and fuel economy of hybrid electric vehicles and plug-in hybrids was approved in July 2010. The recommended procedures for PHEVs were revised at Argonne National Laboratory, and EPA's new regulation to define PHEV fuel economy reporting protocol is expected to be based on SAE J1711.[33][34] In November 2010 EPA decided to rate electric mode and gasoline only mode separately, and these are the two figures prominently displayed in the window sticker of the 2011 Chevrolet Volt. In electric mode the Volt's rating is estimated with the same formula as an electric car.[4][7] The overall or composite fuel economy rating combining electricity and gasoline powered are displayed in the Monroney label in a much smaller type, and as part of the comparison of the Volt's fuel economy among all vehicles and within compact cars.[35] EPA has considered several methodologies for rating the overall fuel economy of PHEVs, but as of February 2011 EPA has not announced the final methodology that will be applied for the purposes of estimating the new manufacture's 2012-2016 Corporate Average Fuel Economy (CAFE) credits for plug-in hybrids.[7][36]

Examples[edit]

In November 2010 the EPA began including "MPGe" in its new sticker for fuel economy and environmental comparisons. The EPA rated the Nissan Leaf electric car with a combined fuel economy of 99 MPGe,[3] and rated the Chevrolet Volt plug-in hybrid with a combined fuel economy of 93 MPGe in all-electric mode, 37 MPG when operating with gasoline only, and an overall fuel economy rating of 60 mpg-US (3.9 L/100 km) combining power from electricity and gasoline.[4][35][37] For both vehicles EPA calculated the MPGe rating under its five-cycle tests using the formula displayed earlier with a conversion factor of 33.7 kW-hr of electricity being the energy equivalent of a gallon of gasoline.[4]

All-electric cars[edit]

The following table compares EPA's fuel economy expressed in miles per gallon gasoline equivalent (MPGe) and out-of-pocket fuel costs for 15 series production all-electric cars for model years 2011-14 rated by the EPA as of May 2014, versus EPA rated most fuel efficient plug-in hybrid, the Chevrolet Volt, and the most efficient gasoline-electric hybrid car, the Toyota Prius third generation.[38][39][40] The table also shows, when available the fuel efficiency expressed as KW-hrs/100 mile, the metric used by EPA to rate electric cars before November 2010.[20]

Comparison of fuel efficiency and economics for all-electric cars rated by the EPA for the U.S. market
against EPA rated most fuel efficient plug-in hybrid and hybrid electric vehicle in the U.S. for model years 2011-14
(Fuel economy and operating costs as displayed in the Monroney label
and the U.S. Department of Energy and U.S. Environmental Protection Agency's fueleconomy.gov website)
Vehicle Model year Type of
powertrain
Operating
mode
EPA rated
Combined
fuel economy
EPA rated
City
fuel economy
EPA rated
Highway
fuel economy
Cost to drive
25 miles
Annual
fuel cost
Notes
BMW i3[41] 2014 Electric car All-electric 124 mpg-e
(27 kW-hrs/100 mi)
137 mpg-e 111 mpg-e $0.81 $500 See (1)
The 2014 BMW i3 is the most
fuel efficient EPA-certified vehicle
of all fuel types considered in all years.[38]
Scion iQ EV[42] 2013 Electric car All-electric 121 mpg-e
(28 kW-hrs/100 mi)
138 mpg-e
(24 kW-hrs/100 mi)
105 mpg-e
(32 kW-hrs/100 mi)
$0.84 $500 See (1)
Chevrolet Spark EV[43] 2014 Electric car All-electric 119 mpg-e
(28 kW-hrs/100 mi)
128 mpg-e
(26 kW-hrs/100 mi)
109 mpg-e
(31 kW-hrs/100 mi)
$0.84 $500 See (1)
Honda Fit EV[44] 2013 Electric car All-electric 118 mpg-e
(29 kW-hrs/100 mi)
132 mpg-e
(26 kW-hrs/100 mi)
105 mpg-e
(32 kW-hrs/100 mi)
$0.87 $500 See (1)
Fiat 500e[45] 2013 Electric car All-electric 116 mpg-e
(29 kW-hrs/100 mi)
122 mpg-e 108 mpg-e $0.87 $500 See (1)
Nissan Leaf[46] 2013 Electric car All-electric 115 mpg-e
(29 kW-hrs/100 mi)
129 mpg-e 102 mpg-e $0.87 $500 See (1)
Mitsubishi i[47] 2012-13 Electric car All-electric 112 mpg-e
(30 kW-hrs/100 mi)
126 mpg-e
(27 kW-hrs/100 mi)
99 mpg-e
(34 kW-hrs/100 mi)
$0.90 $550 See (1)
Smart electric drive[48] 2013 Electric car All-electric 107 mpg-e
(32 kW-hrs/100 mi)
122 mpg-e
(28 kW-hrs/100 mi)
93 mpg-e
(36 kW-hrs/100 mi)
$0.96 $600 See (1)
Ratings correspond to both
convertible and coupe models.
Ford Focus Electric[49] 2012-13 Electric car All-electric 105 mpg-e
(32 kW-hrs/100 mi)
110 mpg-e
(31 kW-hrs/100 mi)
99 mpg-e
(34 kW-hrs/100 mi)
$0.96 $600 See (1)
BMW ActiveE[50] 2011 Electric car All-electric 102 mpg-e
(33 kW-hrs/100 mi)
107 mpg-e 96 mpg-e $0.99 $600 See (1)
Nissan Leaf[51] 2011-12 Electric car All-electric 99 mpg-e
(34 kW-hrs/100 mi)
106 mpg-e
(32 kW-hrs/100 mi)
92 mpg-e
(37 kW-hrs/100 mi)
$1.02 $600 See (1)
Tesla Model S[52] 2013 Electric car All-electric 95 mpg-e
(35 kW-hrs/100 mi)
94 mpg-e 97 mpg-e $1.05 $650 See (1)
Model with 60kWh battery pack
Tesla Model S[53] 2012 Electric car All-electric 89 mpg-e
(38 kW-hrs/100 mi)
88 mpg-e
(38 kW-hrs/100 mi)
90 mpg-e
(37 kW-hrs/100 mi)
$1.14 $700 See (1)
Model with 85kWh battery pack
Toyota RAV4 EV[54] 2012 Electric SUV All-electric 76 mpg-e
(44 kW-hrs/100 mi)
78 mpg-e
(43 kW-hrs/100 mi)
74 mpg-e
(46 kW-hrs/100 mi)
$1.32 $850 See (1)
Coda[55] 2012-13 Electric car All-electric 73 mpg-e
(46 kW-hrs/100 mi)
77 mpg-e
(44 kW-hrs/100 mi)
68 mpg-e
(50 kW-hrs/100 mi)
$1.38 $850 See (1)
BYD e6[56] 2012 Electric car All-electric 62 mpg-e
(54 kW-hrs/100 mi)
60 mpg-e
(56 kW-hrs/100 mi)
64 mpg-e
(52 kW-hrs/100 mi)
$1.62 $950 See (1)
Chevrolet Volt[57] 2013 Plug-in hybrid Electricity only 98 mpg-e
(35 kW-hrs/100 mi)
- - $1.05 $900 See (1) and (2)
Most fuel efficient PHEV
with a gasoline/electricity
combined rating of 62 mpg-e.
[38]
Gasoline only 37 mpg 35 mpg 40 mpg $2.57
Toyota Prius[58] 2010-13 Hybrid electric vehicle Gasoline-electric
hybrid
50 mpg 51 mpg 48 mpg $1.74 $1,050 See (2)
Most fuel efficient HEV,
together with the Prius c.[59]
Notes: All estimated fuel costs based on 15,000 miles annual driving, 45% highway and 55% city.
(1) Values rounded to the nearest $50. Electricity cost of $0.12/kw-hr (as of November 30, 2012). Conversion 1 gallon of gasoline=33.7 kW-hr.
(2) Premium gasoline price of US$3.81 per gallon (used by the Volt), and regular gasoline price of US$3.49 per gallon (as of November 30, 2012).
Plug-in hybrids[edit]

The following table compares EPA's estimated out-of-pocket fuel costs and fuel economy ratings of serial production plug-in hybrid electric vehicles rated by EPA by June 2014 versus the most fuel efficient gasoline-electric hybrid car, the 2010-13 Toyota Prius third generation, and EPA's average new 2013 vehicle, which has a fuel economy of 23 mpg-US (10 L/100 km; 28 mpg-imp).[60] The table also shows the fuel efficiency for plug-in hybrids in all-electric mode expressed as KW-hrs/100 mile, the metric used by EPA to rate electric cars before November 2010.[20]

Comparison of out-of-pocket fuel costs and fuel economy for plug-in hybrid electric cars
rated by EPA as of June 2014 with MPGe and conventional MPG(1)
(as displayed in the Monroney label and the US DoE fueleconomy.gov website)
Vehicle Year
model
Operating
mode
(EV range)
EPA rated
Combined
fuel economy
EPA rated
city
fuel economy
EPA rated
highway
fuel economy
Fuel cost
to drive
25 miles
Annual
fuel cost(1)
(15,000 mi)
Notes
BMW i3 REx[61] 2014 Electricity only
(72 mi)
117 mpg-e
(29 kW-hrs/100 mi)
- - $0.87 $650 The EPA classifies the i3 REx as a
plug-in hybrid while CARB as a
range-extended battery-electric vehicle (BEVx).
Gasoline only 39 mpg - - $2.24
Honda Accord Plug-in Hybrid[62] 2014 Electricity
(13 miles)
115mpg-e
(29 kW-hrs/100 mi)
- - $1.36 $950 The 2014 Accord is the most fuel
efficient plug-in hybrid in
electric/hybrid mode
with a rating of 115 mpg-e.
Electricity
and gasoline
46 mpg 47 mpg 46 mpg
Chevrolet Volt[63] 2013 Electricity only
(38 miles)
98 mpg-e
(35 kW-hrs/100 mi)
- - $1.05 $900 The 2013 Volt is the most fuel
efficient plug-in hybrid car with a
combined gasoline/electricity
rating of 62 mpg-e.[38]
Gasoline only 37 mpg 35 mpg 40 mpg $2.57
Toyota Prius PHV[63] 2013 Electricity
and gasoline
(11 miles)
95 mpg-e
(29 kW-hrs/100 mi
plus 0.2 gallons/100 mi)
- - $1.44 $950 After the first 11 miles the car
functions like a regular Prius hybrid
Gasoline only 50 mpg 51 mpg 49 mpg $1.74
Chevrolet Volt[64] 2012 Electricity only 94 mpg-e
(36 kW-hrs/100 mi)
95 mpg-e
(36 kW-hrs/100 mi)
93 mpg-e
(37 kW-hrs/100 mi)
$1.08 $1,000
Gasoline only 37 mpg 35 mpg 40 mpg $2.57
Ford C-Max Energi[65][66]

Ford Fusion Energi[65][66]
2013/14 Electricity
and gasoline
(20 mi)
88 mpg-e
(37 kW-hrs/100 mi)
95 mpg-e 81 mpg-e $1.36 $1,050 The Energi did not use any gasoline
for the first 20 mi in EPA tests,
but depending on the driving style,
the car may use both gasoline
and electricity during EV mode.
Gasoline only 38 mpg 40 mpg 36 mpg $2.29
Cadillac ELR[67] 2014 Electricity only
(37 mi)
82 mpg-e
(41 kW-hrs/100 mi)
- - $1.23 $1,100
Gasoline only 33 mpg 31 mpg 35 mpg $2.89
Porsche 918 Spyder[68] 2015 Electricity only
(12 mi)
67 mpg-e
(50 kW-hrs/100 mi)
- - $1.50 $2,100
Gasoline only 22 mpg - - $4.33
Fisker Karma[63] 2012 Electricity only
(33 miles)
54 mpg-e
(62 kW-hrs/100 mi)
- - $1.87 $1,750
Gasoline only 20 mpg 20 mpg 21 mpg $4.76
Porsche Panamera S E-Hybrid[69] 2014 Electricity
and gasoline
(16 mi)
50 mpg-e
(52 kWh/100 mi)
- - $3.49 $1,850 The all-electric range is between 0 to 15 mi
Gasoline only 25 mpg 23 mpg 29 mpg $3.81
Toyota Prius[70] 2013 Gasoline-electric
hybrid
50 mpg 51 mpg 48 mpg $1.74 $1,050 Most fuel efficient hybrid electric
car, together with the Prius c.[59]
Ford Taurus FWD[71]
(Average new car)
2013 Gasoline only 23 mpg 19 mpg 29 mpg $3.79 $2,300 Other 2013 models achieving
23 mpg include the Chrysler 200,
Toyota Venza, and
Hyundai Santa Fe Sport 2WD.[71]
Notes: (1) Based on 45% highway and 55% city driving. Electricity cost of US$0.12/kw-hr, premium gasoline price of US$3.81 per gallon (used by the Volt, Karma and 918 Spyder),
and regular gasoline price of US$3.49 per gallon (as of November 30, 2012). Conversion 1 gallon of gasoline=33.7 kW-hr.

Conversion using GGE[edit]

The same method can be applied to any other alternative fuel vehicle when that vehicle's energy consumption is known. Generally the energy consumption of the vehicle is expressed in units other than W·h/mile, or Btu/mile so additional arithmetic is required to convert to a gasoline gallon equivalent (GGE) of 115,000 BTU/mile.[citation needed]

Hydrogen example with GGE[edit]

The 2008 Honda FCX Clarity is advertised to have a vehicle consumption of 72 mi/kg-H
2
.[72] Hydrogen has an energy density of 120 MJ/kg (113,738 BTU/kg),[73] by converting this energy density to a GGE, it is found that 1.012 kg of hydrogen is needed to meet the equivalent energy of one gallon of gasoline. This conversion factor can now be used to calculate the MPGe for this vehicle.

 MPGe = vehicle\ efficiency \times {GGE},
 MPGe = 72 \frac{mi}{kg-H_2} \times {1.012 \frac{kg-H_2}{gallon\  gasoline}} = 72.8

Life cycle assessment[edit]

Pump/Well-to-wheel[edit]

EPA's miles per gallon equivalent metric shown in the window sticker does not measure a vehicle's full cycle energy efficiency or well-to-wheel life cycle. Rather, the EPA presents MPGe in the same manner as MPG for conventional internal combustion engine vehicles as displayed in the Monroney sticker, and in both cases the rating only considers the pump-to-wheel or wall-to-wheel energy consumption, i.e. it measures the energy for which the owner usually pays. For EVs the energy cost includes the conversions from AC from the wall used to charge the battery[30] The EPA ratings displayed in window stickers do not account for the energy consumption upstream, which includes the energy or fuel required to generate the electricity or to extract and produce the liquid fuel; the energy losses due to power transmission; or the energy consumed for the transportation of the fuel from the well to the station.[8][31]

Petroleum-equivalency factor (PEF) — a CAFE metric[edit]

In 2000 the United States Department of Energy (DOE) established the methodology for calculating the petroleum-equivalent fuel economy of electric vehicles based on the well-to-wheel (WTW) gasoline-equivalent energy content of electricity (Eg). The methodology considers the upstream efficiency of the processes involved in the two fuel cycles, and considers the national average electricity generation and transmission efficiencies because a battery electric vehicle burns its fuel (mainly fossil fuels) off-board at the power generation plant.[8] This methodology is used by carmakers to estimate credits into their overall Corporate Average Fuel Economy (CAFE) for manufacturing electric drive vehicles.[7]

The equations for determining the petroleum equivalent fuel economy of electric vehicles are the following:[8]

PEF = Eg * 1/0.15 * AF * DPF
where:
PEF = Petroleum-equivalent fuel economy
Eg = Gasoline-equivalent energy content of electricity factor
1/0.15 = "Fuel content" factor or incentive factor. DoE selected this value to keep consistency with existing regulatory and statutory procedures, and to provide a similar treatment to manufacturers of all types of alternative fuel vehicles
AF = Petroleum-fueled accessory factor
DPF = Driving pattern factor

The gasoline-equivalent energy content of electricity factor, abbreviated as Eg, is defined as:

Eg = gasoline-equivalent energy content of electricity = (Tg * Tt * C) / Tp
where:
Tg = U.S. average fossil-fuel electricity generation efficiency = 0.328
Tt = U.S. average electricity transmission efficiency = 0.924
Tp = Petroleum refining and distribution efficiency = 0.830
C = Watt-hours of energy per gallon of gasoline conversion factor = 33,705 Wh/gal
Eg = (0.328 * 0.924 * 33705)/0.830 = 12,307 Wh/gal
PEF = Eg * 1/0.15 * AF * DPF = 12,307 Wh/gal/0.15 * AF * DPF
PEF = 82,049 Wh/gal * AF * DPF

The petroleum-fueled accessory factor, AF, is equal to 1 if the electric drive vehicle does not have petroleum-powered accessories installed, and 0.90 if it does.

The driving pattern factor, DPF, is equal to 1, as DoE considered that electric vehicles eligible for inclusion in CAFE will offer capabilities, perhaps excepting driving range, similar to those of conventional vehicles.

In the example provided by the US DoE in its final rule, an electric car with an energy consumption of 265 Watt-hour per mile in urban driving, and 220 Watt-hour per mile in highway driving, resulted in a petroleum-equivalent fuel economy of 335.24 miles per gallon, based on a driving schedule factor of 55 percent urban, and 45 percent highway, and using a petroleum equivalency factor of 82,049 Watt-hours per gallon.[8]

See also[edit]

References[edit]

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External links[edit]