Using the manual
http://www.300zx-twinturbo.com/cgi-bin/manual.cgi
you can check the speed sensor. If it's not there work upwards along the wiring. Provided there are no breaks or bad contacts you can probably assume it's a bad or broken connection in the dash unit, which, unless you can find the exact solder joint that's broken or burnt, will require a replace gauge cluster.
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1989 240SX Fastback. . . . . . . . . . . . . . . . . . . . . . . . . . . 1995 300ZX Twin Turbo
Warning: Objects in mirror aren't as fast as they thought they were.