I put the same tires (Nitto 555)on my 91 Z & am having a bad time getting the vibration out. They have balanced the back 3 times & replaced the back tires for me. We balanced the front twice & it seemed to help a bit, between 72 to 78 mph it vibrates in the seat. Over 80 it seems to level out good, have to keep at it I guess. lorne