The video above is a minute with VW's Ecoracer, which is made from carbon fibre and gets 83 mpg. VW has yet to make it, but a car very much like it for sale is the Smart Roadster. Below the fold is a video of Fiat's Ecobasic which has a modular, plastic design and can get 94 mpg. Fiat decided not to the make it but you can feel parts of it are in the Fiat 500.
Why didn't these two companies build these cars? The Roadster sports car, while admired by some hypermilers, is a car for a market that doesn't place fuel economy at the top of the concerns list. The high cost of carbon fibre also plays a role in Fiat's decision, but plastics that could function similarly are slowly starting to make it into car parts. Keep your fingers crossed.