check out the many exhaust posts. I went with a 2 1/2 inch system, flowmaster 50 series, and seemed to improve mid-range power. There also a lot of posts about fuel economy here: aerodynamics, low rolling resistance tires, synthetic oil, driving habits, rear axle ratio all play a role.
1995 F-150, 300-6, 4x4, ex-cab, sb, 3.08 5-speed, 2 1/2 in. exhaust w/flowmaster 50, MSD + ignition upgrades, canopy, 174,500 + miles Gone but not forgotten: 1965 F-100 240 3spd lwb "the green hornet"; 1991 f-150 300 5sp 2wd sb, 1960 F-250 4x4, 223 six, 4sp with wrap-around rear window, two-tone paint.
Tow vehicle: 2012 F-250 4x4 6.7 diesel XLT s/c lwb 20+mpg