With all of the attention being paid to the 230 mpg number that the Chevy Volt will apparently be granted by the EPA, the Automotive X Prize thought it was time to weigh in on the subject of calculating fuel efficiency for vehicles that use energy sources other than gasoline. They don't like it. Instead, the AXP prefers MPGe, a "rigorous and more neutral measure" of fuel efficiency. The AXP's John Shore walked us through how the long-running competition thinks about MPGe. They've been at it for a while.

First, let's define MPGe. MPGe stands for miles per gallon equivalent, and measures fuel economy based on the energy content of a gallon of petroleum-based gasoline. For those who like formulas, the AXP defines MPGe as (miles driven) / [(total energy of all fuels consumed)/(energy of one gallon of gasoline)]). Understanding and using MPGe is important, now more than ever, Shore said, because MPG is no longer particularly useful from the consumer's point of view. "It is obsolete," he said. Shore gave three reasons for moving away from MPG:
  1. The growing popularity of gasoline alternatives. When everyone was using gas (or diesel), MPG offered a decent way to compare the efficiency of different vehicles, especially if one drove prudently and understood how the ratings were calculated. But putting, for example, E85 into the tank changes the whole equation.
  2. We are now seeing vehicles that can use two power sources, most obviously plug-in hybrids that use electricity and a liquid fuel. MPG really loses its meaning when there is more than one fuel and only looking at the liquid fuel allows PHEV advocates to claim they get 100 or 150 mpg, which is kind of true but also deceptive, Shore said.
  3. Lastly, as soon as we introduce plug-in vehicles, the efficiency of the vehicle – no matter how you measure it – becomes a very strong function of how far you drive. This issue is not addressed by MPGe, but should be considered by people who want to understand fuel efficiency better. MPG is MPG no matter how long you drive it for. But a plug in hybrid changes depending how long it's been since the last charge.
Read more after the jump.

[Source: Auto X Prize]


Since MPGe is the "figure of merit" that the AXP is using for the competition, they're confident that it's the best way to compare apples to pineapples. Shore noted that the natural gas industry faced a similar problem when they introduced CNG cars, and also chose to use MPGe (then called GGE, gallon of gas equivalent).

Shore and the AXP realized that most customers don't have any intuition about what kilowatt hours mean, so he added a simple calculation to the spreadsheet they use to calculate MPGe that includes the number of hours you plug in your vehicle and which type of outlet you're plugging into. This allows users to see how long it takes to get a set amount of equilavent energy from a gallon of gasoline. For example, a vehicle plugged into a 110 V circuit for eight hours gets about the same energy as what is in a third of a gallon of gasoline. Shore was surprised by this, and said that, "This shows why gasoline has remained king. in five minutes, you can put a sh*tload of energy into a car."

Shore added that, while the AXP thinks that MPGe is part of the answer, consumers will need to know three things about fuel efficiency numbers in the future: the MPGe, what sort of driving was done to achieve those numbers and, if the vehicle has a plug, how far the vehicle was driven. Armed with this information, Shore said, consumers will be able to accurately compare how efficient different vehicles, no matter what type of powertrains they have.




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