Suburban Modifications

I added a 50 Series Flowmaster muffler with 3" tail pipe, poweraid throttle body spacer and an Airaid intake system to my 1999 2500 7.4L Suburban. It seems to have more power. However, I've always gotten 10 miles to the gallon, now I'm getting about 8.5. (And no, I am not driving more agressivley). I've been told that all the increased air flow is causing the motor to run too rich, and that an adjustment to the computer would likely solve my problems.

Therefore, I am looking a a perfomance chip or tuner. Any opinions out there? Is a custom chip best? Will it do any good? Please help.