How accurate is a GPS speedometer?

Short answer: under an open sky, a modern phone reads speed to within roughly 1 km/h, and often better than that. That is more accurate than the speedometer in most cars. The long answer is more interesting, because how the number is produced matters far more than the hardware.

Two ways to get a speed out of GPS

The obvious method is to take two position fixes, measure the distance between them, and divide by the elapsed time. It works, but it inherits every metre of positional error. If each fix is good to ±5 m and they arrive a second apart, a stationary phone can appear to be doing 18 km/h in a random direction. This is why cheap trackers flicker wildly when parked.

The better method is Doppler. Satellites are moving at about 3.9 km/s relative to the ground, and the frequency of their signal shifts according to how fast the receiver is closing on or receding from each one. With four or more satellites in view, the receiver solves for its velocity vector directly — no positions involved. The result is dramatically steadier, typically accurate to a few centimetres per second, and it is what your phone reports in the speed field of a location fix when it can.

Live Speed Tracker always uses the Doppler figure when your device supplies one, and falls back to the positional calculation only when it does not. The status bar on the live GPS speedometer tells you which one is in use at any moment, because the difference in trustworthiness is large enough that you should know. If it reads a little below your car’s dashboard, that is expected: why your dashboard reads higher.

What the accuracy number means

The figure shown as ±12 m is the horizontal accuracy radius: the W3C spec defines it as the circle, centred on the reported position, that contains your true position at 95% confidence, but Android reports a 68% radius and browsers pass it through unchanged, so treat it as a rough guide rather than a guarantee. It says nothing directly about speed accuracy — but the two correlate, because the same poor satellite geometry that widens the position circle also degrades the velocity solution.

As a rule of thumb:

• Under ±10 m — good. Trust the reading.
• ±10–25 m — fair. Fine for a general sense of speed; not for timing runs.
• Over ±25 m — poor. This page discards those fixes entirely rather than showing you a number it does not believe.

What degrades a reading

• Buildings. Urban canyons reflect signals, and a reflected signal arrives late, which the receiver interprets as extra distance. This is called multipath and it is the single biggest source of error in cities.
• Tunnels and covered parking. No signal at all. Your speed will freeze, then jump when you re-emerge.
• Tree cover and deep valleys. Fewer satellites in view means worse geometry.
• A cold start. With no recent almanac data, a receiver can take 30–60 seconds to get its first fix. It is not broken; it is looking.
• Where the phone sits. A metal dashboard, a phone in a pocket, or a heated windscreen with an embedded element all attenuate the signal. A windscreen mount with a clear view of the sky is measurably better than a cup holder.

If the speedometer is stuck at 0 or says location is blocked, the problem is a permission, not accuracy.

Why this page filters

Three filters run on every fix before it reaches the display:

1. Fixes with an accuracy radius worse than 30 m are discarded outright.
2. Any reading implying more than 400 km/h is treated as a GPS jump and thrown away.
3. Distance only accumulates while you are genuinely moving, above about 1.8 km/h.

That third one matters more than it sounds. Without it, a phone left on a parked dashboard for an hour will happily record two kilometres of travel from pure jitter — and every downstream statistic, including your average speed, is then wrong.

The count of filtered fixes is shown in the status bar. A handful over a long drive is normal. Dozens means you are somewhere with poor sky visibility, and you should treat the numbers with suspicion.

Sources

• GPS.gov: GPS Accuracy gives the official civilian figure: a smartphone is typically within a 4.9 m radius under open sky, and worse near buildings and trees.
• W3C Geolocation API defines coords.speed and coords.accuracy, the two fields this page reads for speed and the accuracy radius.
• ESA Navipedia: GNSS Basic Observables explains the Doppler observable, from which a receiver derives its velocity directly rather than from successive positions.