Dude, there's no "speed" you shouldn't go over, but driving for an extended period of time at 90mph with the O/D off (should have put you somewhere about 5000 RKM) probably isn't a very good idea. For all intensive purposes, you shouldn't turn off the o/d when you're on the highway.

1999 Silver Frost SVT #609 of 2760 Quaife, lightened SVT Flywheel, SPEC stage II clutch, removed resonator, k&n drop in - various other goodies too.