The Dashboard Measures Wheel Motion, Not Ground Speed
A car’s speedometer usually estimates speed from how quickly its wheels rotate. The car’s computer knows the expected distance covered per rotation, then converts rotations into miles per hour. That estimate depends on the tires having the size the system expects and rolling as intended.
So a dashboard reading of 70 mph does not reveal an exact road speed. Tire pressure, tread wear, replacement tires of a different size, and changes to the wheels can all alter how far the car travels with each rotation. If a tire’s effective circumference is smaller than the car expects, it turns more times over a given distance and can make the indicated speed higher than the actual speed.
Why the Reading Often Runs High
Many speedometers are calibrated conservatively, so they tend to avoid showing a speed below the car’s true speed. That means a driver seeing 70 mph may actually be moving somewhat slower—but there is no universal amount to subtract. The reading varies by vehicle, tire condition, and calibration, and some cars may be closer to accurate than others.
A GPS speed display estimates motion differently, using changes in the vehicle’s position. At a steady speed with a good satellite signal, it can help compare against the dashboard, though it may lag during acceleration or lose precision in poor reception. The only way to know how far a particular car’s reading differs is to check that car under steady conditions.
