How to measure vehicle speed?
3 Answers
Speed is calculated by the formula V=S/t, which represents the distance traveled per unit of time. The speedometer uses sensor data to monitor the number of wheel rotations, wheel diameter, and time to calculate the actual vehicle speed.
Last time I drove my friend's old Santana, I looked into this. The traditional method is to check the dial next to the tachometer on the dashboard—when the needle points to 60, that's 60 km/h. Nowadays, new cars mostly have digital displays, and you can see the real-time speed at a glance in the bottom right corner of the center console screen. By the way, when using mobile navigation, it also shows the speed, though with a slight delay. Actually, the car calculates speed mainly based on the number of wheel rotations, with a small device near the transmission specifically counting this. If you're curious, you can try it on an open road—when using cruise control, the data on the dashboard is especially steady. I noticed an interesting thing with Gaode Navigation: when GPS signal is weak in a tunnel, the speed display freezes.
From a tech enthusiast's perspective, this is pretty cool – the car's onboard computer converts wheel speed sensor pulse signals into speed values displayed on the dashboard. In daily life, the most practical feature is actually the speed measurement function in mobile navigation apps, which uses satellite positioning to calculate coordinate displacement differences divided by time, achieving over 98% accuracy. For more professional applications, differential GPS equipment is required, capable of reducing errors to centimeter levels. Once at a track day, I saw a modified car enthusiast connect a racing dashboard via the OBD port, with a real-time refresh rate three times faster than the factory dashboard. When driving on the highway at night, I sometimes estimate speed by observing the spacing between streetlights – if the standard interval is 50 meters, passing 4 streetlights in 3 seconds roughly translates to 60 mph. Some high-end dash cams now also come with GPS speed measurement, allowing playback videos to display speed curves from the recording.
