Only true in cars that can't take advantage of higher octane. In modern computer controlled engines with knock sensors, premium will usually provide slightly better mpg's, but not necessarily enough to offset the higher cost of the gas.

Of course, premuim is recommended in our Mini's.

 

Uhh in cars with VVT higher octane=better MPG. hence why people an get rediculous MPG on the way home from a track day. 100 octane race fuel is awesome.