In a jet engine, each major rotating section usually has a separate gauge devoted to monitoring its speed of rotation. 
Depending on the make and model, a jet engine may have an N gauge that monitors the low-pressure compressor section and/or fan speed in turbofan engines. The gas generator section may be monitored by an N gauge, while triple spool engines may have an N gauge as well. Each engine section rotates at many thousands RPM. Their gauges therefore are calibrated in percent of a nominal speed rather than actual RPM, for ease of display and interpretation.