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does a car speedometer measure velocity
The speedometer of the car shows the speed of 100 kilometers per hour, and the navigation shows the speed of 96 kilometers per hour. Why are the two displayed speeds inconsistent? In fact, the speed of the car instrument and the speed of the navigation are not absolutely accurate. Both have some degree of error. For example, when taking a curve, the speed displayed by the navigation is inaccurate, because the GPS locates a straight line, but unable to locate the curve! The speed of the instrument panel will be affected by the vehicle tires and other aspects, and the speed of the navigation display will also be affected by the signal. The two displayed speed values can never be the same, because the calculation principle is completely different. However, if the navigation signal is stable, the displayed value is the most accurate. The speed measured by navigation is based on the actual speed of the car on the map. Data reporting is intuitive.
From this point of view, the speed reported by the navigation system is more accurate than the speed displayed on the instrument panel. However, considering the speed of the navigation satellite speed signal, there are some errors with the actual speed due to time delay, measurement errors, map accuracy and other reasons. When the GPS prompts that the speed is 100km, you will find that the pointer of your dashboard speedometer has exceeded 100km. That is to say, when the pointer of the speedometer points to 100km, your actual speed has not reached 100km. It may look like 95-97km. As for the difference between the two, the difference will be different due to different car enterprises and different models. Under normal conditions, the difference between the vehicle speed displayed on the instrument panel and the actual vehicle speed is about 2~7km/h. So the speedometer shows 100km/h, and the actual speed is about 93km/h-98km/h. Generally, there are three sensors on the vehicle that can obtain wheel rotation data. One is the input shaft and output shaft sensor on the gearbox, and the other is ESP. The speed is calculated by calculating the speed of the drive shaft, and then the result is calculated. It will be sent to the CAN bus and finally reflected on the vehicle's instrument.
Generally, the speed indicated on the instrument panel by car manufacturers will be 2-4km/h faster than the actual speed. However, due to different design standards of various vehicle enterprises, as well as factors such as tire pressure and wheel wear, the final indicated speed is slightly deviated. Speed is a measure of distance over time. But the car speedometer can't actually measure your speed from point A to point B. The automobile speedometer usually works by measuring the rotation of the automobile drive shaft, axle or wheel. Then they use some basic mathematics to infer the rotation and determine your speed.
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