While John is right that MPGe (miles per gallon equivalent) is a flawed way to indicate the efficiency of plug-in vehicles, he's wrong to blame it on the EPA and I also disagree with using miles/kWh. 

EPA labels actually carry MPGe ratings because the techno-dolts in congress mandated it. The 2007 energy bill that raised fuel economy requirements to 35 mpg by the end this decade also specifies that new vehicles MUST be rated in miles per gallon. The only way to do that for vehicles run on non-liquid fuels is to calculate the equivalent based on the energy content of the fuel.  For an explanation of what MPGe means, check out this video Because You Asked…What is MPGe or Miles per Gallon Equivalent?

As for what should replace MPGe, miles/kWh is also flawed and has the same problems as miles per gallon. The relationship between mileage and energy consumption is reciprocal and as mileage increases, the energy savings actually decrease. For example going from 20 to 40 mpg actually only saves half as much fuel as doubling from 10 mpg to 20 mpg and going from 40 to 80 mpg cuts the savings in half again. 
http://green.autoblog.com/2009/07/23/greenlings-where-are-the-most-important-mpg-increases-at-the-u/

What we should be using is kWh/100 miles so that we actually measure consumption in a linear fashion. 

Reshared post from +Green Car Reports

What's the best way to measure the efficiency of a plug-in electric car?