The equivalent in terms of greenhouse-gas emissions or MPGghg explained:
Having an EV we do not completely eliminate CO2 emissions. All depends on how we generate the electricity used to recharge the battery packs. Electricity arriving at the charging station can be produced either by means of renewable, nuclear, hydro or through more common sources still present in many other countries: coal and gas. From coal and oil as primary sources for electric power generation, any electric vehicle will behave like a 29-MPG gas car. Using solar power gets you a "350-mpg" car, while wind power gives you 2,500 MPG car. If the power generation is hydro-based that is equivalent to a 5,100-MPG gas car. Usually, all this electric power generation sources are in a mix. In the United States, as now in 2017, the average EV consumption gets 55.4 MPGghg or we can state is equivalent to a 55.4 MPG gas car. All depends on how much green is the electrical greed.
Because electricity varies as price, very expensive in Honolulu, circa 25c per kWh and very cheap in Louisiana circa 7c per KWh, we have another conventional measuring unit called eGallon. eGallon is the cost of fueling a vehicle with electricity compared to a similar vehicle that runs on gasoline. US average for regular gasoline is one gallon for $2.5 and an electric eGallon for $1.20. $540 spend on eGallons in a year time will correspond to an average of 15,000 miles per year. To evaluate the efficiency of an EV's follows the numbers.