I think fuel efficiency has been maxed out on our current metal cars. Thats why most we can get is 30-40mpg for a car of any brand or origin. Unless youre willing to drive one of these, which can get better mileage at the risk of safety
Basically we need an automotive revolution. Use of lightweight composites like plastic and carbon-fiber for body panels. A lean burn engine, probably made of ceramics. Id say 100mpg is easy. Thing is, that technology is available now ^^