signal wire

hey i have a 94 cavy with a 2.2l and i recently bought a tach and i hooked it up to the signal wire or a wire that came off the distributer and everything, when i start my car the tach reads 500rpm and thats normal idling rpm but when i rev the engine up to about what sounds like redline the tach reads 3000rpm and i know thats not right. so i figured that some how that signal wire can't read the rpm fast enough or something like that, could anyone help me fix it.

thanks

sounds to me like its only reading half the rpms, i dont know about cavaliers made before 95 but on the ones made after 95 i know you need a tach adaptor for it to work right, UNLESS you can set your tach to 2 cylinders, then that would work also