Originally posted by cojones:
short gears: no effect
R&P: may drop it by some irrelevant amount (mathematically its about 4% -- changing tire diameters would give you about the same variance)
Uh... no.
First, let's talk about top speed. The chart in the FAQ shows the theoretical speed at redline in each gear, not the top speed that's actually attainable in each gear without exceeding redline. In all gears other than top gear, those two are the same thing. However, the NSX cannot reach redline in the top gear because it is drag-limited. It will top out at 168 mph, short of redline.
Now, let's talk about acceleration at various speeds. Acceleration is indeed affected by the R&P and the short gears. At some speeds, the modded gears will be faster; at other speeds, the stock gears will be faster.
Let's take a stock five-speed NSX and a five-speed NSX with 4.23 and short gears as an example. This gearing is shorter than stock in every gear. Thus, assuming that torque is roughly constant (it's close enough that that's a valid assumption for analytical purposes), acceleration will be faster at any speed where you are in the same gear, either way. For example, at 0-43 mph, the modded car will be quicker due to its shorter gearing (with the R&P). At 43 mph, the modded car will need to upshift; at 43-45 mph, the stock car will still be in first gear and will accelerate more quickly than the modded car. Above 45 mph, both cars are in second gear, and again, the modded car is quicker, until it needs to upshift to third at 70 mph. At 70-81 mph, the stock car accelerates faster because it's still in second while the modded car is in third. The stock car will also accelerate faster at 99-114 mph, and 133-144 mph; the modded car, at other speeds.
Depending on which road speeds you're looking at, the modded car may indeed be slower than the stock car. As you can see, the speed range of 99-114 mph is in the heart of the speed you were traveling, and could have easily affected your acceleration, since you could have stayed in third gear with the stock gears, but were forced to fourth gear with the modded setup.
Per Bob Butler, here are the calculated times in seconds for the individual components of the speeds traveled, first for a stock five-speed '91, then for a similar car with short gears and 4.23 R&P:
0-90 mph: stock 10.16, short 9.68
90-100 mph: stock 2.14, short 2.36
100-110 mph: stock 2.41, short 2.71
110-120 mph: stock 3.45, short 3.08
90-120 mph: stock 8.00, short 8.15
These times include the 0.3 seconds to accomplish each shift.
As you can see, when accelerating from 90 to 120 mph in each case, there is one upshift from third to fourth. The stock gears take 8.00 seconds to accelerate from 90 to 120, and the short gears take 8.15.
The advantage of the stock gears is even more pronounced when accelerating from 90 to 110, because the stock gears don't need to upshift. The stock gears take 4.55 seconds, and the short gears take 5.07.
Each gearset has advantages at different speeds. In general, the short gears have the biggest advantage in the early part of second gear (40-70 mph), whereas the stock gears have the advantage at higher speeds, such as the situation presented here. The R&P may help or hurt, depending on where it requires an additional upshift.
