It is a daily driver, smogged etc, is why I am leaning to 3.73's for a 06 4.6 auto, CAI w/ a 91 Bama/XCT/Mustang Evo tuneYou will see a difference in mpg. Just to give you an idea. When my son burns a tank in his 2000 GT I still have 1/4 tank of gas left in my 1999 GT.
At Project Pony - in my 69 Mach I, I installed a bullet proofed AOD behind a 427 stroker (500+ rwhp) and changed the rear to a 4.56 Detroit Locker. After factorying in the final drive ratio I ended up with basically 3.73 in overdrive which is very reasonable on the highway, but it's nasty stop light to stoplight. ������ Tons of fun to drive.
I know we're talking about different years and hardware, but I think you will be very pleased if you use the 4.56 setup (provided this is not a daily driver).