Idle speeds vary from car to car, but for a passenger car, typical idle speeds are between 500 and 900 rpm. This idle speed also depends on the accessories that are currently running as a car's idle control system will automatically raise the idle in order to provide power for auxiliaries such as air conditioning and power steering.
An engine idle speed is set by the manufacturing in order to ensure the best performance and life of the engine. If idle speed is too high, then the engine's economy will suffer, as the fuel consumption will be increased. This also means that there will be higher emissions and environmental side effects, as well as an increase in engine wear. Consequently, if the idle speed is set too low, then engine damage can occur from auxiliaries such as the water pump and oil pump not operating effectively enough to maintain proper cooling and oil lubrication, respectively. In addition, the engine itself may not be producing enough power to overcome its own frictional losses, and thus will rough roughly or stall. According to About, a good idle is typically an indicator of engine health, and if there is anything wrong under the hood, then idle quality is likely to suffer first.