For gasoline engines the signal source is the ignition system; the signal is called the "TD" pulse. When the signal source is via a magnetic pickup at the crankshaft (as in the example above) the signal frequency is 1 pulse per revolution of the crankshaft. When the signal source is the ignition system of a gas engine it generates a frequency that is a function of the number of cylinders in the engine. A four cylinder gas engine will generate 2 pulses per revolution, a six cylinder 3 pulses per rev, an eight cylinder 4 pulses per rev, etc.

As noted above, obviously a tach displays crankshaft speed.