I have a 2000 GT with 98000 miles on it; I am the original owner. Had the oil changed and fuel system cleaned about a month ago. At that time, I was getting 21 MPG, and the fuel system cleaning was supposed to (possibly) add 2 MPG. Since then, I have dropped 1 MPG from each successive tank of gas I have put in...down to 17 MPG now. I removed the air induction assembly to check for any rags they might have left in it when they "checked" the air filter (which is only 3 months old and not dirty)...but there were no air flow restrictions. The engine runs smoothly still, and has as much or more power than it did before the fuel system was "cleaned". Tires are still inflated to the same PSI. I plugged my OBD II computer into the care, and there are no codes, either active or stored. What could be causing this problem???