I agree with the not stepping up to 3.55, but 3.73s would do wonders. I put gears in my 99 GT auto a few weeks after I bought the car and it made a big difference. With a converter shift kit & cooler to compliment the gears, the car was a rocket off the line and pulled much harder to 130 mph. At 75 mph, the rpms will be between 2500-2600 rpms. With the stock gears, the rpms at 75 are about 2250-2300 rpms. If you do change the gears, you definitely need a chip. I would recommend a JMS(Johnson Motorsports) chip.
WWW.JMSchip.com. Talk to Steve and I'm sure they can set you up with a more aggressive tune for power, shift points & converter lock up which will make a big difference. Start saving for a quality converter. I recommend either a Precision Industries Stallion converter or Protorque converter. I have used both and both are excellent.
www.converter.com www.protorque.com
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99 Silver GT
4R70W, ProTorque TC(3200), Transgo, FMS Cooler, Synth. ATF & 3.73 gears
Mac L/T, Cat-H, Borla Cat-back
V2 S-trim, AFR p/p, 70mm t/body, Pro-M80, 42# inj., 255lph pump, JMS chip
D&D K, A-arms, Coil-overs, KB Super Subs, Mac Adj. UCA/LCA, MM C/C Plates