Working out how many miles a given "energy load" will push you is a pretty good rule of thumb, and as you point out, no gasoline car gets 120 mpg. I suspect the efficiency factor you mentioned is part of this discrepancy.

On the other hand, it is possible to get 40 miles from one gallon of gas, which is still a pretty good number for gasoline. On the gripping hand, it's been calculated that the cost of charging an electric car comes to about 75 cents compared to one gallon of gasoline. But then there's battery replacement...

A lot of tradeoffs, no easy answers.

Put the cars out there, and let people decide for themselves which answer is best.