For the graph “Oil's Advantage Drops,” which shows EROIs over time, the data was from papers cited above. Conventional oil: Guilford et al., 2011. California heavy oil: Brandt, 2011. Soybean biodiesel: Pradhan et al., 2011.
The numbers in the graphic “Mileage Return on Investment,” are based (for all the liquid fuels) on a fairly simple equation from unpublished work by Carey King of the University of Texas at Austin. Multiplying the EROI by the car's mileage (in miles per gallon), then divided by the energy density of the fuel (in gigajoules per gallon) gives the miles you can travel, for one gigajoule of input into making the fuels. I used an average mileage of 30 miles per gallon for new gasoline cars and 33 miles per gallon for diesel cars based on EPA fuel economy estimates. (Note that King's calculations used somewhat different EROI values than I did, so the numbers for “mileage return on investment” differ somewhat from his results.)
For electric cars, I also used EPA estimates of the miles they can go per kilowatt-hour of electricity input, multiplied by the EROI for average U.S. electricity—a weighted average of all the sources of electricity in the U.S., based on EIA statistics for electricity generation. This estimate for electric cars does not include the energy required to manufacture the cars and their batteries—nor does the estimate for conventional cars include the energy required to make them. There is a good reason to think that electric cars require more energy to create, because the battery production process does require some additional energy to build. Life cycle analyses, however, such as Notter et al., “Contribution of Li-Ion Batteries to the Environmental Impact of Electric Vehicles,” Environmental Science & Technology, (2010), estimate that part of the energy required to make the battery is offset because electric cars have a simpler drive train, which requires less energy to build. Overall, the energy required to make an electric car is around 20 percent greater than that needed to make a conventional car, Notter et al. estimated. I hope that researchers will publish results of more comprehensive estimates for “mileage return on investment,” or some similar measure for transportation services that factors in the EROI of different energy sources.
The author thanks Charles Hall, Adam Brandt, Carey King, David Murphy, Matthew Heun and Michael Dale for interviews, checking calculations and offering information. Any errors remain the responsibility of the author.