I remember some people talking about this a long time ago, check it out:

http://www.compgeeks.com/details.asp?invtid=TACHOMETER

Software is for windoze, but I'm sure that it wouldn't be too hard to hack
something up for Linux.

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Nate Carlson <natecars at real-time.com>   | Phone : (952)943-8700
http://www.real-time.com                | Fax   : (952)943-8500