How GPS speedometers work — and how accurate they are

Where the number comes from

iOS combines satellite signals (GPS and other GNSS systems) with other sensors to produce location updates, each with an estimated speed and a horizontal accuracy. ClearSpeed reads those updates while it's open, converts the speed to your unit, and displays it.

What affects accuracy

• Sky view. Tall buildings, dense trees, bridges, and tunnels block or reflect signals.
• Placement. A phone deep in a bag, a footwell, or under a metallic windshield tint sees less sky.
• Low speed. At walking pace, small position errors are a big fraction of the movement, so readings wander more.
• Just after launch. The first fix can take a few seconds, longer without cellular data.
• Device. Wi-Fi-only iPads have no GPS receiver and usually cannot report speed on the move.

How ClearSpeed handles weak signal

• A status pill shows Good GPS with the reported accuracy, Weak GPS with a reason, or Searching.
• If reported accuracy is worse than about 30 m (≈100 ft), or a reading is stale, the app shows Weak GPS instead of a speed number.
• Displayed speed is smoothed ("Balanced") so it doesn't flicker, and jumps are limited to what's physically plausible.
• Trip distance ignores implausible position jumps, and trip time pauses while the signal is unusable.

How accurate is it?

With a clear sky and steady movement, phone GPS speed is typically close to true speed — often closer than a car's dashboard, which is usually calibrated to read slightly high. We don't publish a precision figure because it depends on your device and conditions. Treat the number as an informational estimate, not a certified measurement.