What is the most fuel-effiecient automobile in the world?

masterdeath01 asked:

I live in the United States (but any calculation will do… either MPG or KPL).

I want to know the most fuel-efficient vehicle in the entire world to date that is available for purchase in the United States (so I could drive it, obviously). I’m not looking for concept cars or automobiles that need to be bought as far as India or something. I live in California… you get it.

Electric vehicles, Hybrid Vehicles, Diesel-powered Vehicles, and etc. are welcome.

Thank you, all.