Had you changed out the intake prior to changing the exhaust? I saw a drastic reduction in mileage when they were both changed out - you're basically increasing both inflow and outflow, like going to a larger diameter water pipe. By increasing the airflow, the engine can burn fuel more efficiently - meaning more gas consumption. Since your computer is still thinking it's balancing the old equation, things should be out of wack until it "learns" the new setup. Resetting the should help, but you may never get back to the original fuel efficiency - it's the price you pay.

-- John