need help tach

i already wired an aftermarket tachometer and connected the green wire from the tach to the white wire in the engine. the tach works but it jumps all over the place and never stays still, i dont know whats wrong any help is appreciated>

what white wire did you hook it to. and i suppose the green one is for the rpm signal right??.....im just gonna start sayin random stuff that will make sense when you see it.... dont hook it up to the wire hooked onto the ignition(i think its white and i think thats what you hooked to, but its no biggie just throw electrical tape around it itll be fine) go to your car and look under the hood, look at the back on the firewall on the left hand side, you will see a thick bundle of wires wrapped in black tape runnin into the wall. follow that bundle up 8 to 10 inches, youll see a connector taped to the bundle, its gonna be blue or brown, thats the correct rpm signal wire. the wire is a striped white and black i believe. see if that does the job. let me know if that doesnt work.

i tried looking for the connecter but i couldn't find it. i have a 2001 mirage 1.8 is that the car you are talking about? and how come it says i have to connect it to the ignition failure sensor (whatever that is)