i have a 90 coupe been swap from 4 cyl to 8 cyl i put a 5 inch autometer tach in it and it reads out wrong it readin about double the rpms it should. what could cause this? it doesnt a have a switch for a certain number of cyl. or ay extra wires only the red, white, green, and black

 

Hook the tach signal to the V8 ignition box. The 4cyl will have a higher pulse rate, double in fact, since it has half the number of cylinders.