What could cause my tach to read double on my 91 Z28?? I changed the cluster to a 145 speedo and the same problems. It also causes my gas gauge to act funny anything over a half tank and it reads full... I think its a bad ground, I looked but found nothing..I need the help here guys, any suggestions??

Thanx John

 

maybe if u were taking the tach from an engine with a diff # of cylinders then yours,

 

Maybe you have wires open to ground and/or crossed.