as long as ive had my iroc the tach has indicated the wrong rpm. i dont know where to start . at idle it indicates about 1200-1300 but when i check it with a meter its really only turning about 750 rpm. the more rpm the engine turns the more its off. any ideas what might be wrong. the gauge cluster is 110 mph cluster and i dont know if its original.

 

I have a 91 RS with a v6 cluster and at idle the tach says 1200 and the tach also says i shift at 7000. Was pretty cool until i install a aftermarket tach and i really shift at 4800.