Final answer:
The red light in an Over Speed Indicator illuminates when the engine speed exceeds a certain threshold.
Step-by-step explanation:
In an Over Speed Indicator, the red light typically illuminates when the engine speed exceeds a certain threshold. This threshold is set by the manufacturer and can vary depending on the specific vehicle and its intended use. For example, in a sports car designed for high-performance driving, the red light may come on at a higher engine speed compared to a standard passenger car.