Although no one currently makes a totally new gearset (cheaply), you can swap different 5th gear sets from other models.

When mildly modded, the early 1986-1988 FC3S could easily attain about 160mph in 5th gear at redline.  You can swap in an FD3S 5th gear, which will push 5th gear redline speeds up to around 180mph.  By using the taller 5th gear, you also gain slightly better gas mileage cruising on the freeway.

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