When the speedometer shows 130, what is the actual speed?

[JulianaAnn]

The speedometer shows 130 km/h, but the actual speed does not exceed 120 km/h. Experimental speed measurement: The speed displayed on the car dashboard is not accurate and has a certain discrepancy compared to speed radar measurements. Someone once conducted an experiment where the dashboard showed 130 km/h, but the speed radar only registered 117 km/h, which did not exceed 120 km/h. Therefore, this would not be considered speeding. How the speedometer works: A car has a speed sensor that outputs a square wave frequency signal. Inside the dashboard, there is a microcontroller that calculates the speed by counting the pulses using an internal counter. The microcontroller then drives a stepper motor through a driver module, which moves the speed pointer you see. Although this method of measuring speed is relatively accurate, it still has some inaccuracies compared to professional speed radar. Of course, this is not meant to encourage speeding on highways. Since the configurations of different cars vary, generally speaking, the speed displayed on the dashboard is slightly lower than the actual speed. This design has significant benefits because when the dashboard shows 120 km/h, the actual speed is only around 110 km/h, encouraging drivers to maintain a safe speed and avoid speeding.

[DelElliana]

This issue was verified during the last test track session. Simply put, when your dashboard shows 130km/h, the actual speed is approximately between 122 to 128km/h. This discrepancy exists because automakers must comply with national standards that mandate the displayed speed can only be higher than, not lower than, the actual speed. During my test, I used a professional GPS speedometer to measure three popular sedans consecutively. When maintaining a displayed speed of 130km/h on straight roads, the actual speeds generally fluctuated around 125km/h. However, note that if you switch to non-OEM specification tires, the error margin increases. For instance, bald treads or overinflated tires will make the displayed speed even more inaccurate. Generally, for every 3mm reduction in tire tread depth, the actual speed becomes about 1km/h slower than the displayed speed.

[MacHaven]

As a traffic regulation researcher, I pay close attention to this point. The law stipulates that the indicated speed on the vehicle dashboard must be higher than the actual speed, with the error margin required to be between +10% and +4% of the current speed under standard load conditions. When the dashboard displays 130km/h, the actual speed never exceeds 125km/h (lower error limit). For example, in last year's national standard tests, 30 mainstream models showed an average actual speed of 123.6km/h at a displayed 130km/h. Interestingly, due to more responsive instantaneous torque display, electric vehicles have about 1.5% smaller error margins compared to fuel-powered cars. During regular calibration, it was found that a 0.3bar decrease in tire pressure increases the speedometer overreading by 0.8km/h.

[DylanAnn]

Veteran driver tip: When the speedometer shows 130 km/h, the actual speed is about 125 km/h! During long-distance driving, I specifically compared the car's navigation system with mobile GPS and found that the speed displayed on the dashboard is consistently 4%-6% higher. Once, when driving a friend's car with modified rims, it was even more exaggerated—the dashboard showed 130 km/h while the GPS only reported 118 km/h. For safety, it's best to drive 5 km/h below the displayed speed. For example, if it shows 130 km/h, drive at 125 km/h to ensure you don't exceed the speed limit. If you've recently changed tires or rims, remember to visit a shop to recalibrate the speed sensor parameters using a diagnostic tool; otherwise, the error could be even greater. Be extra cautious on downhill slopes, as gravity can increase your actual speed by about 3 km/h compared to flat roads.