I have installed the K&N air filter and I see an increase in gas mileage. Before the change I would see 48-49 MPG on my drive to work. Since the change I see 52-54 MPG. It must be making more power, but I could not quantify how much more. It probably accelerates quicker, but is still not quick. I had an '07 MAZDASPEED 3 before the iA so it seems very slow to me.