Abstract: An non-intrusive tachometer for measuring engine RPM includes a hand-held probe having an antenna for sensing radio frequency electromagnetic signals radiated from an operative spark ignition engine. The probe includes means for providing a pulse signal indicative of plug firing events to signal processing means which executes a statistical algorithm to determine the time between spark plug firings and engine RPM. A visual display displays engine RPM. Automatic gain contol means provides the pulse signal with a relatively constant amplitude by compensating for the inherently wide variation in amplitude of the electromagnetic signals among different automobile ignition systems. The signal processing means and display are located separate from the probe in an emissions testing system, or the signal processing means and display are integrated within the probe.