We don't have numbers, so this is all speculation. Still, I think we can agree it's a neat idea that definitely deserves exploration.

With regards tot he transmission question: my car (and many others) allow me to select the gear even though it has an automatic transmission. In my car it's called Autostick, other cars it will be something else; sportshift maybe.

In this way, I can ensure the gear will be optimal for gasoline consumption.

Also, perhaps in the future, a model can be made to incorporate regenerative breaking.