The 1999 GT's came from the factory with 3.27 gears if I'm not mistaken, 3.55 gears won't really make that much of a difference in my opinion, I would recommend going with the 3.73 gears. I went from the 3.27's to 3.73's in my 1996 and am glad I did. The increase in acceleration was very noticeable without sacrificing the top end. My car with the T-45 transmission tach's about 2500 RPM at 75 MPH's. Hope this helps.