How to Wire a Tachometer | It Still Runs Every engine spins within the limits of its design. The pistons inside the engine pump the crankshaft to spin. This spinning crankshaft sends horsepower to the street. A tachometer counts the number of rotations the crankshaft is making a minute (RPM). It can also be How to Connect a Tachometer to the Chevy 350 | Our Pastimes Jul 30, 2018A tachometer is a tool which measures speed in revolutions per minute, or rpm, at which an engine shaft is rotating. A tachometer is usually found on the dashboard of a car, but you you can buy a separate one to measure your engine's rpm if needed. If your Chevy 350 engine has a broken tachometer you can connect a new one in your vehicle.