Have you ever encountered this situation on the highway: the navigation system indicates a current speed of 80 km/h, but the speedometer shows 90 km/h; or you steadily maintain 90 km/h, only to find other cars easily overtaking you.
Is your car being too “modest,” or is the speedometer “kindly deceiving” you? Don’t underestimate this seemingly insignificant difference. Understanding the distinction between “displayed speed” and “actual speed” can not only help you avoid speeding tickets but also improve your driving safety.
Why does the speedometer display a higher speed?
This is not a manufacturing error but stems from mandatory national standards. Regulations clearly stipulate that the speedometer display must be greater than or equal to the actual speed, and is absolutely not allowed to be lower than the true value. For example, when the actual vehicle speed is 90 km/h, the speedometer can display between 90 and 100 km/h, but it must never be lower than 90 km/h.
In other words, making the speedometer “run faster” helps us control our speed more reliably, avoiding unknowingly exceeding the speed limit due to an underestimation, thus reducing safety hazards.
How to estimate actual speed?
Typically, the speedometer reading will be about 5-10 kilometers per hour higher than the actual speed. That is, when the speedometer shows 90 km/h, the actual speed is approximately 80-85 km/h. Therefore, when you control your speed at around 85 km/h on the speedometer, the actual speed may only be around 90 km/h.
What other influencing factors are there?
Besides design standards, the following factors can also further increase the speedometer reading error:
First, tire condition: Replacing tires with smaller sizes or chronically underinflated tires will cause the tire diameter to decrease and the number of rotations to increase, resulting in a higher speed reading on the speedometer.
Second, vehicle load: When fully loaded with passengers or cargo, the tires deform under pressure, reducing the contact radius with the ground, which may also make the displayed speed appear “inflated” than usual.
Finally, road conditions also play a role: When going uphill, the engine load increases, and the displayed speed may be higher than the actual speed; while going downhill, due to inertia, the actual speed will be closer to the speedometer reading.
What’s the use of understanding this speed difference?
Understanding this difference first helps us control our speed more accurately. For example, on a road with a 90 km/h speed limit, if the speedometer shows 90 km/h, the actual speed may only be 85 km/h, perfectly within the legal range.
Secondly, it helps in the timely detection of potential vehicle problems: If one day you notice a significant difference between the speedometer reading and the navigation speed, for example, the speedometer shows 120 km/h but the navigation reports 105 km/h, be alert for potential tire leaks, severe wear, or transmission system malfunctions, and have it checked as soon as possible.
Finally, understanding speed measurement logic is also important: Traffic management departments use speed measuring equipment (including section speed measurement) based on the vehicle’s actual speed, not the speedometer reading. Understanding this helps you drive more confidently while remaining compliant with regulations.
The slightly higher-than-actual speed on the speedometer is actually a “safety buffer” provided by engineers.
Understanding the underlying principles and logic allows us to drive with greater confidence and precision.
Next time you’re driving on the highway, pay attention to the differences between the navigation prompts and the instrument panel display. You might gain a deeper understanding of your car’s “personality”—safety is always hidden in the details.
