This is great reading guys but now i wonder if simply changing out second gear wouldn't achieve everything needed?
If you only change out your second gear, then you are shortening the gap between first and second, but lengthening the gap between second and third. By changing out gears 2 through 4, the wider spacing is spread out more (between third and fourth, and between fourth and fifth).
Has anyone ever found a legitimate reason why Japan changed the gear ratio for the US market?
I am only guessing, but I can think of two possible reasons. They may have done it to increase the car's fuel economy numbers. You'll recall that somewhere around that time, for the same reason, Chevrolet had a design "feature" on the Corvette in which the automatic upshifted from first to fourth unless you were really hammering the throttle. The other possibility is that they were optimizing the car's performance on the racetrack by having second through fifth, the gears used on the track, closely spaced, and having a big gap between first and second, where it doesn't matter.
Where did these figures come from?
They come from an extensive analysis that Bob Butler performed and published in a series of articles published in NSX Driver, the magazine of the NSX Club of America. The model, algorithms, and equations he used were explained in the article. He compared acceleration figures for each 10 mph interval (as well as for 1/4 mile acceleration) for the various gearsets and the various models and years of the NSX, as well as for hypothetical horsepower additions and weight reductions. The numbers produced by his model correlated just about perfectly with actual test results performed by the major automobile publications.
And yes, the stock gears really ARE over three seconds faster from 70 to 150, and the short gears really are 0.3 second faster from 0 to 70. Look at the gearing differences and you'll see why (numbers below assume the 4.062 R&P in both cases):
Up to 45 mph, both sets have the same gearing
From 45 mph to 73 mph, the short gears are 13 percent shorter
From 73 mph to 81 mph, the stock gears are 23 percent shorter
From 81 mph to 101 mph, the short gears are 14 percent shorter
From 101 mph to 114 mph, the stock gears are 19 percent shorter
From 114 mph to 139 mph, the short gears are 7 percent shorter
From 139 mph to 144 mph, the stock gears are 25 percent shorter
Above 144 mph, both sets have the same gearing
So the short gears are a little bit shorter (7-14 percent shorter) at speeds in which both gearsets are in the same gear, but the stock gears are a whole lot shorter (19-25 percent shorter) at speeds in which the short gears force you into a higher gear while the stock gears keep going up to a higher speed in the lower gear.