What do you want to know here, the difference or what. Could not get the question.

WHAT YOU WANT TO ASK

If you are asking about their use or definitions, then a tachometer is a dial next to the speedometer on the instrument panel which measures the rotation of the engine's crankshaft. A defogger is a device which is used for clearing the condensation from the windshield.

Defogger: Used for clearing up the glass, a system of electrical copper wires attached on the rear windscreen is the defogger, as the name suggest, it removes the fog.

Tachometer calculates the rpm(x1000) of the car at a particular point.