I have a 2006 GT vert with a magnacharger running 10 psi, long tube and manaflow exhaust and auto. I picked up about 1 mpg with the supercharger as long as I drive normally. I have about a 45 mile commute one way. I'm guessing the improvement in mileage came from the tune and the magnacharger has a bypass valve so they claim the supercharger has almost no drag at cruise.