I've seen where several people have asked for GM Tach Filters. As far as I know the oem is not available any more. I have found some information about making a replacement. It's really simple and inexpensive. I bought the parts from an electronics supply store for under a dollar, and it took less than ten minutes to make. Here's the schematic. If you have any questions, ask.

What does this do for you?

What does this do for you?

I thought these were for using a 6 cylinder tach on a V8 engine so it reads properly and gives a faster response when you rev the engine. Not 100% sure though.

Here's your answer .
http://www.egauges.com/vdo_send.asp?Sender=Classic_Tach_Filter