punto GT.
most turbo engines are fairly efficiant.
e.g a 2.0 16vT saab that will eat a lot of road cars for breakfast, will manage 23mpg around a track on a track day, and upto 41 on an open a road, 36mpg is the norm
HOWEVER a 3.0 audi A6 avant quattro, will manage 29mpg on THE VERY BEST of days, thats like with quality petrol in the tank, one person driving, and a 70mph tail wind
and around 18 on a bad one.
and its painfully slow compared to saab, well down on torque compared to the turbosaab too
std 2.0 16v NA will usually bang out around 34-36mpg, more if your good, less if your not, and it will also be painfully slow.
Think of it this way, with BIG capacatiy NA engines you are using pretty much teh same fuel no matter how your driving it and how many revs' throttle aperture load etc.
Smaller capacity yet turbo'd, its only using that extra fuel on boost to prevent pinking and engine meltdown, so off boost its getting the same economy or BETTER (since they are usually lower compression etc) than its equivilent capacity NA counterpart.