I have to imagine that the greatest gains would be in having a complete engine shutoff during stops and slow speeds. Some of the hypermilers who use the engine shutoff during stops/idles could probably give the best estimate for gas savings for that aspect. I'm not sure how much more could be gained by a full hybrid system.
Personally, I'd much rather convert an old 'stang to full electric. As I remember, the aerodynamics aren't great, so it would make a much better weekend/neighborhood cruiser than a highway runner anyway.
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