Understanding MPG and MPGe

Erik Kountz
December 19, 2016

Submitted as coursework for PH240, Stanford University, Fall 2016

Introduction

Fig. 1: The EPA fuel efficiency sticker for an electric vehicle. [3] Notice that the sticker gives fuel efficiency in MPGe and in kilowatts per 100 miles. Additionally, the CO2 emissions are calculated from the tailpipe only so any CO2 emitted in producing the electricity is not counted. (Courtesy of the DOE)

Fuel efficiencies of vehicles are measured in miles per gallon or MPG. But what exactly is MPG, and how does it compare to other common measurements of fuel efficiency such as L/100km or MPG equivalent (MPGe)? How does fuel efficiency compare to CO2 emissions? This report will briefly describe what MPG and MPGe do and do not measure and how they compare to other measures of fuel efficiency.

Explanation of MPG

Most simply, MPG measures how many miles a vehicle can travel with one gallon of fuel. The larger the value of MPG, the more fuel efficient the vehicle is. The same idea applies to the metric equivalent of km/L. The conversion between the two is:

Miles per Gallon (MPG) = 2.350 × Kilometers per Liter (km/L)
Kilometers per Liter (km/L) = 0.425 × Miles per Gallon (MPG)

These equations use the US gallon, not the imperial gallon. However, many non-US countries use L/100km, the number of liters of fuel needed to drive 100 kilometers. It is related to MPG by

Miles per Gallon (MPG) = 235
Liters per 100 Kilometers (L/100km)
Liters per 100 Kilometers (L/100km) = 235
Miles per Gallon (MPG)

The MPG Illusion

L/100km and other volume/distance units of measure have the advantage over MPG of avoiding the "MPG illusion." The problem is that improvements in fuel efficiency are not linear with increases in MPG while they are linear with decreases in L/100km. One will save more fuel switching from a 10 MPG vehicle to a 20 MPG vehicle than one will save by switching from 20 MPG vehicle to a 50 MPG vehicle. [1,2] To help reduce the problem of the "MPG illusion," the US Environmental Protection Agency (EPA) now requires vehicles sold to include gallons per 100 miles (or kilowatt hours per 100 miles or kilograms of hydrogen per 100 miles) in addition to MPG or MPGe. [3]

MPG equivalent

While the use of MPG and other measures of fuel efficiency work well for liquid fuels, MPG is not a useful measurement for electric, compressed natural gas, liquid natural gas, or hydrogen fuel cell vehicles since those sources of power are not sold by the gallon. This makes it difficult to compare fuel economies of vehicles using alternative fuels. This problem is resolved by introducing a gasoline gallon equivalent (GGE) or diesel gallon equivalent (DGE) - defined to be the amount of an alternative fuel needed to equal the amount of energy in one gallon of gasoline or diesel, respectively. The definitions used by the United States for fuel economy purposes are shown in Table 1.

Fuel Type Gasoline gallon equivalent
(GGE)
Diesel gallon equivalent
(DGE)
Gasoline 1 gallon 0.887 gallons
Diesel 1.13 gallons 1 gallon
Compressed Natural Gas
(CNG)
5.66 pounds (2.57 kg) 6.38 pounds (2.57 kg)
Liquid Natural Gas
(LNG)
5.38 pounds (2.44 kg) 6.06 pounds (2.57 kg)
Hydrogen 2.20 pounds (1.00 kg) 2.49 pounds (1.13 kg)
Electricity 33.7 kilowatt hours (121 MJ) 38.1 kilowatt hours (137 MJ)
Table 1: Amount of various alternative fuels equal in energy to one gallon of gasoline or one gallon of diesel (GGE or DGE). Units are first given in the official government defined units and then in SI (metric) units. [4-8]

Once these conversions are taken into account, it is simple to calculate the MPG equivalent (MPGe). The vehicle takes the same standardized test to calculate MPG as a gasoline or diesel vehicle would take using the alternative fuel.

Limits on MPGe

While the use of MPGe helps compare fuel efficiencies between vehicles, it has some limitations. The first limitation is that MPGe, like MPG, suffers from the "MPG illusion." So a better unit might be GGE per 100 miles or gasoline liter equivalents per 100 kilometers. The second limitation is that calculating the CO2 emissions for an alternative fuel vehicle becomes difficult. The EPA only includes tailpipe emissions for vehicles. [3,9] This is not much of a problem for gasoline or diesel fuel because of their energy density compared to production, transportation, and storage emissions. However, CO2 emissions from electricity needed to compress natural gas, keep liquid natural gas cold, or separate water to make hydrogen, or charge a battery are significant and are not included. Depending on the location and type of fuel, it is possible that using an alternative fuel vehicle will produce more CO2 emissions or use more energy than a similarly sized gasoline or diesel fueled vehicle. [9,10]

Conclusion

In summary, the use of MPGe to compare fuel efficiencies is a positive development, but MPGe still generates the "MPG illusion," and it misrepresents CO2 emissions by only counting tailpipe emissions.

© Erik Kountz. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.

References

[1] E. Chemi, "Not All Fuel Efficiency Is Equal: Understanding the Miles-Per-Gallon Illusion," Bloomberg, 14 Jan 14.

[2] R. P. Larrick and J. B. Soll, "The MPG Illusion," Science 320, 1593 (2008).

[3] "Revisions and Addition to Motor Vehicle Fuel Economy Label; Final Rule," U.S Federal Register, 76 FR 39478, 6 Jul 11.

[4] "Fuel Properties Comparison," U.S. Department of Energy, 29 Oct 14.

[5] "Natural Gas Metering - Utility Metering vs. DNG Dispenser Metering," Clean Vehicle Education Foundation, 22 Sep 11.

[6] T. Butcher et al., "Specifications, Tolerances, and Other Technical Requirements for Weighing and Measuring Devices," U.S. National Institute of Standards and Technology, NIST Handbook 44-2008, October 2007

[7] "Household Vehicles Energy Use: Latest Data and Trends," U.S. Energy Information Administration, DOE/EIA-0464(2005), November 2005.

[8] S. C. Davis, S. E. Williams and R. G. Boundy, "Transportation Energy Data Book: Edition 35," Oak Ridge National Laboratory, ORNL-6992, October 2016.

[10] D. Heinz, "Carbon Emissions From Electric Passenger Vehicles in the United States," Physics 240, Stanford University, Fall 2014.

[11] "Clean Cities Alternative Fuel Price Report," U.S. Office of Energy Efficiency and Renewable Energy, July 2016.