First off, I drive a 2005 Nissan 350Z. I took my car into an auto place a few months ago to get my radiator flushed because I havent had it done since I bought the car with @ 32,000 miles, had it flushed at 50,000. The day before I had it flushed I changed my oil as well. Ever since I took it to the auto place I have only been getting about 17.5-18 MPG. Previously I was getting around 21 MPG. A friend said that it could be the summer heat that was causing this but I've kept track of my MPG for 2 years every time I filled up and in previous summers, its never dipped below 20 MPG. I've changed my oil once again since then but it had no effects. Anyone have any ideas what could have dropped my MPG that much? I mean 3 MPG per tank, have around a 17 gallon tank, I'm losing 51 miles per tank. If gas on average is 3.30 for premium, I'm losing $15 every time I fill up!!