too expensive FE ideas?

[bzipitidoo]

Auto manufacturers do latch on to the FE ideas that are cheap. I'm wondering about the ones they don't use because they're too little return for the expense, and just where is that line?

And some ideas get ignored because styling and marketing are more important, or people don't want them. Aerodynamics seems to be the number one sacrifice in pursuit of convenience and looks. As in, no rear wheel skirts.

But the sorts of things I'm thinking about are things such as ceramic coatings, cryo treatments, lighter components made of magnesium alloys or carbon fiber, instant engine on/off capability, and better transmissions. Conventional automatic trannies are a crime. The manufacturer bears none of the fuel cost for those, and I'm guessing of all the ways to avoid having to shift gears, the conventional automatic is the cheapest from the manufacturer's view and the worst for FE.

So what all is out there that works except it's too expensive to be worth doing? Stuff that would take 500,000 miles or some such to pay back.