People used to think the Z-28s had a speed limiter around 165 even if you ordered Z-rated tires (110 mph limit if you didn't). Turned out 165 was just where 5th hit the red-line limiter and turned the engine off. 6th was just too tall a gear to pull at that speed. Part of the problem with figuring this out is its tough to find a road you can do that one and, when you do, look down at the gauges at that speed. Turns out 2 GIs had taken their Z-28s to Germany and were running the autobahn every day into work. They had us drooling as they tried to answer this questions for drivers back on this side of the pond. I would bet this is the situation with the GTO