Tach Gauge

i had a after market tach in my 86 gt t tops, i took all the intake apart and cleaned it and replased the all the gaskets. i then got a new tach and now for some reason when i have it on the 8 cylinder setting it doesnt work right, it revs to slow for the shift light and when i rev the tach to like 5000 rpm it sound like the engine is really running at like 6500 rpm when i put it on 6 cylinder its better but it ses my engine is idling at like 1500 witch i know its not its at like 900 at the most. any help thank.

May be a bad tach. If its new see if you can swap it out.