I have a 2001 GT convertible I just spent $4,000 on a top-end rebuild. My gas mileage went from 16 + 17 down to 13 and 14 miles per gallon. I've been reading a lot of posts and I have new spark plugs new air cleaner I have cleaned the mass air flow sensor I have put in all four new O2 sensors. I have checked and there are no brake drag issues. I'm wondering if when they put the new timing gear and chain and tensioners on they might be off a gear or two. Would that cause bad gas mileage. Car runs like a beast and idles very smooth