i have a 2002 gt auto looking to do a 6 speed auto swap.. has anybody done this yet or am i wishfull thinking? reason for doing swap is i run 4.30 gears and would like to get better fuel economy when i'm not racing.
What 6-speed auto are you referring to exactly? Have you looked at the individual gear ratio's between the two transmissions? More gears doesn't necessarily mean better mileage.
I haven't looked into it at all, but I hear the new 6R80 transmission is doing great in the new 5.0s. I'd love to have a couple of extra gears just to help in around-town everyday driving, along with extra performance at the track, plus the reduction in drivetrain loss. However, I can't imagine it being worth the cost and hassle just to gain that little bit of performance.
And also, you spoke of fuel mileage. While you may gain a little from reduction of drivetrain loss, you wouldn't see anything from the overdrive gearing. The 4R70W (what you have now) has a .70:1 OD ratio, while the 6R80 has a .69:1 OD ratio. In other words, no noticeable difference.