Ir you want RPM for jsut troubleshooting or occassional reference, don’t spend the money on any tachometer systems. If you have a laptop or Pocket PC like a Dell or HP handheld, you can download a free program called RPM from sound. http://tunelab-world.com/rpmsound.html It works on any internal combustion engine and is accurate to about 1 rpm. I have used it on a light plane to check the accuracy of the mechanical tach. Works great.