Modern cars typically determine their speed using a device called a speedometer, which is part of the car's instrument cluster. The speedometer provides a visual display of the car's current speed to the driver.
Traditional speedometers work based on a mechanical system. A cable connects the transmission or the wheels of the car to the speedometer. As the car moves, the cable rotates at a speed proportional to the car's velocity. The rotating cable is connected to a set of gears, which then drive the needle on the speedometer dial to indicate the current speed.
However, many modern cars now use electronic speedometers instead of mechanical ones. In these systems, the car's speed is determined by sensors and data from various components within the vehicle. Here's a simplified overview of how it works:
Wheel Speed Sensors: Most cars have wheel speed sensors installed on each wheel or axle. These sensors typically use magnetic or Hall effect principles to detect the rotation of the wheels. They send electrical signals to the car's electronic control unit (ECU) or a dedicated speedometer control module.
ABS System: In cars equipped with an anti-lock braking system (ABS), the wheel speed sensors are also used to monitor the rotational speed of the wheels. The ABS system continuously measures and compares the speed of each wheel to detect any wheel slippage or potential lock-up.
ECU or Speedometer Control Module: The ECU or speedometer control module processes the signals received from the wheel speed sensors. It calculates the vehicle speed based on the rotational speed of the wheels. The module may also incorporate other inputs, such as transmission data or engine RPM, to refine the speed calculation.
Display: The calculated speed information is then sent to the car's instrument cluster, where it is displayed on the speedometer dial for the driver to see. In electronic speedometers, the display is usually presented digitally, but it can also mimic the appearance of a traditional analog gauge.
It's important to note that the speedometer in a car is calibrated to provide an accurate representation of the vehicle's speed. However, factors such as tire size, wear, and variations in the manufacturing process can affect the precise accuracy of the speedometer reading.