for most non-hybrid cars the most inefficient driving is stop and go, for electrics, the most inefficient is highway. that is why hybrids are so popular, drive it anywhere anytime and get pretty much the same mileage.
part of the problem of your idea, though i like serial hybrids, is that it all comes down to the efficiency of the conversion. PHEV that have a manual trans and start stop ability are going to be the most efficient.
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