I'm sure I saw on TV a diesel Vauxhall test development car with 2 turbos and it could get 100 mpg! and that was around a race track. If this is because it had 2 turbos, why are manufactures not putting 2 turbos in as standard? Cost?
 
Diesel M5 - erm, nope.
Magic twin-turbos - sequential and parallel turbos have been used on many cars before. It won't give you more mpg however...