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RPM trouble

my rpm gauge is going crazy. it is running higher than normal, but the engine is staying the same. what is causing the rpm to move like this. also when i turn the car off, the rpm gauge goes all the way to the hgihest number, it use to go to zero

my rpm gauge is going crazy. it is running higher than normal, but the engine is staying the same. what is causing the rpm to move like this. also when i turn the car off, the rpm gauge goes all the way to the hgihest number, it use to go to zero

car, engine, mileage?
So the tachometer is reading incorrectly? In modern cars, the tach is fed by the computer.