tach problems

I put a Trisco tach ($25) in my 1990 Chev truck that has a 350 motor. I had the tach on 8 cyl mode but it was giving me high readings, about 600 rpm too
high at 60 mph and the needle bounces around abit. Put the tach in my daughters car (1994 Sundance) and it works fine.
Do some tachs only work on certain vehicles?
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To be more specific some tach's only work with certain ignition systems.
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Mad-Dog
'79 Chevy K-10
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$25 For a tach? Like I said before,buy a good one.
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