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MPGe is based on energy equivalents (thus the e).
1 gallon of gasoline is 115,000 BTU.
1 kWh of electricity is 3,412 BTU.
So a gallon of gas is the equivalent of 33.7 kWh. That means the Tesla battery pack holds the energy equivalent of of 1.57 US gallons of gas.
EPA says the Tesla can go 240 miles on a charge (at a steady 55 mph it will easily do that). That is "1.57 gallons" equivalent, so a crude calculation gives 152 MPGe. I've seen numbers like 135 quoted.
At this point everyone starts talking about "well to wheels". Yeah, there are energy costs to producing the electricity. But there are also huge energy costs to extracting, refining, and delivering a gallon of gasoline. From what I can tell an honest well-to-wheels calculation is actually more favourable to the electric car!
So it's a hugely apples-to-oranges comparison. You're right that MPG for an electric car is pretty foolish, but they have to put something on the stickers that the average consumer can understand.
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