Car speedometers usually estimate speed from wheel rotation and vehicle systems. A GPS speedometer estimates movement from location updates. Both can be useful, but they are not measuring the same source.
Why a car speedometer can read differently
Dashboard speed can be affected by tire size, tire wear, manufacturer calibration, and vehicle settings. Many dashboards are designed to avoid under-reporting speed, so they may show a number slightly higher than a GPS-based reading.
Why GPS can read differently
GPS depends on signal quality and movement over time. It may be less useful at very low speeds, immediately after starting, in tunnels, around tall buildings, or when your iPhone has a weak view of the sky.
How to compare readings
- Compare during steady movement, not while accelerating or braking.
- Use an open area with better sky visibility.
- Watch the ClearSpeed GPS signal indicator before trusting the number.
- Expect small differences instead of perfect agreement.
Use ClearSpeed GPS as an informational display
ClearSpeed GPS is designed to show a clear GPS speed reference. It is not a certified legal instrument and does not replace safe driving, posted signs, or your vehicle's required instruments.