TXCatfish

Please help. I've spent all weekend trying to replace my digital rpm shift light with a Autometer 6874 playback tach but the rpms are lower then what the stock tach shows.

There's not a 4cly switch and there are not any wires looped on the back. It has a red, white, black, green, blue and gray wire. I'm using the wire wire pin #10 on the bottom of pcm.

Any help would be appreciated. Thanks!