That being the case on the new tires I would give them some time to break in before making any judgments on their effect on FE. What are the specs on the tires that are on the car now (front and back) and what did you replace with them?
Check this thread out about weight reduction in the car...read down through the posts to see what others have seen in FE in regards to weight...
http://ecomodder.com/forum/showthrea...tion-3482.html