The mpg loss is negligable although it might be slightly different for an auto. A couple of guys around here with 5 speeds say they actually get better mileage around town as you're upshifting sooner and tend to cruise at a lower rpm range. Ask around but i've heard a lot of members with auto's going with 4.10's on their 99 and up GT's and they have completely streetable cars with decent mileage.
Hopefully next year i'll have 3.73's and a hiflow hpipe on my 96.
Brett
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66 Tahoe Turquoise Coupe
Laser Red 96GT Ragtop
30 Years & 30 inches apart in the driveway!