They are interchangeable. I had to use a 6-spd shift knob on my Celica GT for awhile cause the previous owner stole it or something. I have a 5-spd shift knob right now and will soon by buying a TRD shift knob (or something similar).
yes it is the same pattern for threading...but the 5 speed will have the GT's transmission style on it and the 6 speed is going to have the GTS's transmission style on it.
CelicaHobby.com is an independent Toyota Celica enthusiast website. CelicaHobby.com is not sponsored by or affiliated with Toyota Motor Sales, USA, Inc. in any way. The Toyota and Celica names and logos are trademarks owned by Toyota Motor Sales, USA, Inc.