[Piwoslaw]

As KITT222 mentioned, a turbo gives you more power from a smaller engine. If you just slap on a turbo, then at best you won't notice a difference in fuel consumption (mostly since hypermiler stay in low rpms while turbo kicks in at higher rpm), but if you replace your engine with a smaller turboed unit, then you'll notice an improvement. This is called downsizing and auto companies are starting to get the hang off it. A smaller engine uses less fuel than a larger one (in most cases), plus it's lighter, requires a smaller starter motor, and so slightly smaller battery, etc. Downsizing from a 2.0l to 1.6l turbo you may save as much as 50kg (110lbs) in weight, and maybe 5%-10% in fuel consuption.