It's all about the ratios of the gears in the trasmission, not in the rear end.
The 370z is geared for good acelleration off the line to get good 0-60 and quarter mile times and it is geared for progressive shift points to get you to its terminal velocity of ~185 mph. Those gears are meant for the street.
Most track cars and motorcycles are geared for shift points that roughly go 60, 80, 100, 120, 140, 160 mph, because until you get to the elite classes, 160 mph or so is all you will see on most tracks. For example, at Portland International Raceway, I could get my Aprilia RSV to trap 165mph at the end of the front straight.... and they run indy cars at that track.
The 370z is geared for 42, 69, 99, 126, 160, 188 mph. (I calculated 6th gear at 7,000 rpm, the rest at 7,500). If you give it, say 4.0 rear end gears, you'll get shifts points of 39, 64, 91, 116, 148 and 185 mph. in that scenario, you've made 1st gear useless and 6th gear still gives you the same top speed, just at 7,500 rpm instead of 7,000.
The real solution would be to rear gear the entire tranny.
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