Short answer: a modern phone GPS speedometer is surprisingly accurate — usually within about 1 km/h of your true speed when you have a clear view of the sky. Here's why, and how to get the most accurate reading.
How GPS measures speed
There are two ways to work out speed from GPS, and the good ones use the better method:
- Distance over time — measure how far you moved between two points and divide by the time. Simple, but small position errors add noise.
- Doppler shift — the GPS receiver detects tiny frequency changes in the satellite signals caused by your movement, and reads your speed directly. This is far more accurate and is what phones use when they report speed.
The GPS Speedometer uses the phone's direct speed reading (the Doppler method) when it's available, and falls back to distance-over-time only if it isn't.
So how accurate is it, really?
In good conditions — open road, clear sky, steady movement — GPS speed is typically accurate to about ±1 km/h, which is better than many car dashboards. Studies and everyday use both back this up: for measuring how fast you're actually going, a phone does a genuinely good job.
What reduces accuracy
GPS relies on a clear line to several satellites, so accuracy drops when that's blocked:
- Tunnels and underground car parks — signal is lost entirely.
- Dense cities with tall buildings — signals bounce, causing errors.
- Heavy tree cover or inside a metal-roofed vehicle.
- The first few seconds before the receiver locks on.
- Standing still — tiny GPS drift can look like a crawl of 1–2 km/h.
A good speedometer handles the last one by ignoring movement smaller than the GPS error, so it shows a clean 0 when you're parked.
How to get the most accurate reading
- Use it outdoors with a clear view of the sky.
- Give it a few seconds to lock on before trusting the number.
- Keep the phone where the sky isn't blocked (near a window, not deep in a bag).
- Make sure high-accuracy location is enabled on your phone.
The takeaway
For real-world use, a phone GPS speedometer is accurate enough to trust — often more so than your car's dashboard, which reads high by design. Just give it open sky and a moment to lock on. Try it free on the speedometer: it runs in your browser and your location never leaves your device.