Even before the advent of partially- or fully-electric cars, it was becoming increasingly apparent that the old fuel economy metric of miles per gallon isn't as useful for measuring energy consumption in vehicles as when it was first codified in the original Corporate Average Fuel Economy standard in the 1970s. That is due in part to the proliferation of new fuels--E85, LPG, LNG, CNG, methanol, and hydrogen--but also because expressing the relationship between distance and volume in this way obscured the diminishing returns to higher levels of fuel economy. As a Wall St. Journal column earlier this week put it, adding electricity into the mpg mix, "risks giving consumers inaccurate information about the financial and environmental costs of driving." But if we need a new metric, what should it measure?