A tachometer is used to measure the RPM of an engine. The tachometer can be analog or digital, with most models being analog. The output is usually a number multiplied by 1000, to display the RPM as a lower number than the whole value (i.e. 5.5 for 5500 RPM). Tachometers will usually have a redline, which is over the recommended RPM limit of your engine.