Bad Gas Mileage after 90,000 mile svc and timing belt ???

I have a 99 Accord LX with 106,000 miles on it. I got my 90,000 mile service and timing belt replaced at 100,000 miles. I used to get between 420-440 miles on one tank. Ever since the service I only get 360 or so. Anybody got any ideas? I took it to the dealer and they couldn't find anything. They reset my computer and said that would fix it and it has not...