Here's a free program that'll turn your sound equiped laptop into an acoustic tach. It works well for my airplane engine with straight pipes and can figure out the idle speed of my truck. It cannot track the engine speed of the truck while driving down the highway, partly because my tires are too loud. It will work if you put it near the tailpipe, though. It works best if you limit the RPM range. I set it for 1500 to 2500 and got good results.
The program says it's for Windows 95 and 98. It works on windows NT 4.0, but does not exit cleanly. I needed to reboot afterwards. John (Thanks to Ken Wyatt) http://www.wwnet.net/~rscott/rpmsound.html
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