What year car? A 240z? If so, is it hot out? The tach in 240zs are known to malfunction when it gets hot (95+) outside. I believe it is the resistance (?) loop on the back of the tach which causes the problem. Under normal operation, this loop heats up due to the current passing through it.
When it gets hot outside, the heat is unable to dissipate as well and the tach starts to creep up over time. Eventually it will just peg at redline.
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