[NachtRitter]

Simply adding a turbocharger to an existing engine will probably not do anything for FE. However, putting a smaller engine with a turbocharger into the a vehicle that had a larger 'normally aspirated' engine can help (for example, putting a 4cyl turbo engine into a vehicle that previously had a V8 NA engine) could give an improvement in FE.

"Real" example:
- 2012 BMW 328i 2.0 4cyl Turbo 6spd: 240hp, 23mpg city, 34mpg highway
- 2010 BMW 335i 3.0 6cyl NA 6spd: 230hp, 18mpg city, 28mpg highway

(I put "real" in quotes because I don't know for sure if the aero, gearing, weight, tires, etc are equivalent between the two different model years, all of which can impact the EPA ratings... and all of the 3-series BMWs have turbos in 2012 while none did (in the USA at least) in the 2010 year... but the the comparison should be relatively close... 1 liter less but with turbo gives more HP and better FE for the newer model vs the older model with the larger NA engine)

Of course, as with most things, 'it depends' ... a large car with a turbo engine that is sized too small could cost FE since the driver would be inclined to floor it more often just to get going.