Added Magnaflow duals - Do I need a retune?

[ponyexpress]

I added a Magnaflow dual exhaust system to my 2008 4.0. My cousin who did the work at his very successful muffler shop said that I wouldn't need to have the computer flashed to compensate for this addition. However, since the new muffler system has been installed, my fuel efficiency has dropped 2.3 mile per gallons.

The car runs fine other than this drop in fuel efficiency.

Your thoughts?