Generally tachometer is a gauge to measure rotation speed of any rotating things. In RC hobby tachometer is the instrument for mesauring and monitoring propeller revolutions per minute (RPM). This tachometer has some additional functionality like: thrust, pitch speed and power estimating. I would like to emphasize word “estimating”, because accurate computation above mentioned values is a complex function of many factors, so there are not any simple methods for precise calculation.
Tachometer using Nokia 5110 LCD - [Link]