sorry for posting this but i cant seem to find the thread i was looking for. i remember reading some time back that turboing your car can have gains in mpg under certain conditions? if i recall correctly under cruising speeds or something.

is this possible and if it is how does this work?

 

Negative. In general a turbo will decrease efficiency slightly when not in boost because of the additional restriction in the exhaust (although you may not even notice a change in terms of MPG), and obviously in boost you will use a lot more fuel.

The only way mileage could increase from turboing would be because the new tune used with the turbo is more efficient at typical cruising RPMs, but the same gain could be had with a proper NA tune.