Dual exhaust for MPG?!? I'm goin to say "try the friggin search button" at this point in the discussion
But If I didn't want to pay for a new car and title and crud, I would swap the rearend gears and put in a stickshift all with the lowest ratio I can find, but no way I would pay someone to do it. Fortunately a lot of performance bent folks will just about give that stuff away.