A tachometer blank the blank at which an engine runs.

Answer: A tachometer measures the speed at which engine runs.

Explanation:

In the first blank space there should be a verb - measures and the second blank space is referring to what is measured, that is the speed.

A tachometer or revolution-counter is measuring the rotation speed and it is an instrument. The speed is shown on the display.

''Tachometer'' is the word that is considering Greek words such as tachos which means speed in Greek and metron which means measure. It is device used in automotive world and the other vehicles that has speed and speed measuring.