TACH READING is a term used to describe the process of measuring the speed of a rotating object, such as an engine or motor. The two main components of this process are REVS (4 letters) and RPM (3 letters). REVS stands for revolutions per second, which is the number of times the object rotates in a given amount of time. RPM stands for revolutions per minute, which is the number of times the object rotates in one minute. TACH READING is an important part of engine maintenance and performance tuning.