As with all forced induction applications, it is going to decrease gas mileage. But depending on how much boost your run also affect gas mileage. But for example roots always has boost even at idle. Depending on how you tune a turbo and size it, also Centrifgal, you can get better gas mileage around town if its not in boost basically. Since you set it up so the turbo spools at around 3000 rpm, so around town as long as you are under 3k rpm, you are basically driving around N/A car. Similar w/ CSC except because its affected more by engine rpm not too much better. Hope that helps.
ADTR 1999 Black Bonneville SE
2003.5 Yellow Mazdaspeed Protege
2003 Azure Blue Mach 1 Mustang