A GPS speedometer is generally more accurate than a mechanical or factory dashboard speedometer under most driving conditions. When a GPS device has a clear view of the sky, its speed reading is typically accurate to within 0.1 to 0.2 mph, while a vehicle’s built-in speedometer often intentionally overreports your speed by 2 to 5 mph.
The primary differences in how they function impact their accuracy. Mechanical & Electronic Vehicle Speedometers
Traditional mechanical speedometers use a physical cable attached to the transmission, while modern electronic ones use computer sensors to track wheel rotation. Both share the same fundamental flaw: they do not measure how fast your car is moving across the earth—they only calculate how fast your tires are spinning.
Intentional Overestimation: By law in many regions (such as Europe and the UK), a vehicle’s speedometer must never underreport actual speed. To protect themselves from liability and protect drivers from speeding tickets, manufacturers calibrate dashboards to read 5% to 10% higher than your actual speed.
Tire Dynamics: Speedometers assume your wheels are a strict, factory-calibrated size. If you install larger or smaller wheels, or if your tires experience natural tread wear, under-inflation, or temperature fluctuations, your speedometer’s math becomes incorrect. GPS Speedometers
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