How Accurate Is Your Phone's GPS Speedometer?

A phone GPS speedometer uses satellite signals to calculate your position roughly once per second, then works out speed from how much your position changed. It's quite accurate out in the open, but error can grow depending on your surroundings.

What affects accuracy

• How much sky is visible: Between tall buildings, in tunnels, or in dense forest, signals bounce or drop out and your position jumps around.
• Speed: At slow speeds like walking, position error (usually a few meters) is large relative to the distance covered, so speed readings are jumpy. The faster you go, as on a bike or in a car, the more accurate it becomes.
• Right after starting: Just after you open the app, it may not have locked onto enough satellites yet, so the first few dozen seconds can be off.

How to reduce error

1. Before you start, let the GPS lock on for about 30 seconds somewhere with a clear view of the sky.
2. Keep the phone somewhere facing the sky, like an armband or mount, rather than deep in a bag.
3. Look at average speed and total distance rather than instant speed to reduce the impact of spikes.

When your top speed looks suspiciously high

If your position jumps sharply once between buildings, the app may calculate that you "moved tens of meters in a few seconds," logging an unrealistic top speed. Good GPS apps filter out these spikes, but they're hard to eliminate completely. If a top-speed record looks off, check the route map for signs of a position jump on that stretch.