Hey Doozle, The 3.90 gears come behind a 6 speed manual trans with a 3.00 first gear---the auto has a 3.58 first gear which means you'd be in first gear for about 7 feet--which would give you 4 more gears basically making the auto a 4speed auto--also the Diablo tuner will not let you tune that combination. Hope that helps--cfan