Written by Muneeb Rana · Last updated 26 October 2026
Quick answer: GPS speedometer accuracy is usually good at steady speeds in open sky: phones measure velocity from the Doppler shift of satellite signals, which is more precise than position. Accuracy drops in tunnels, dense cities, under heavy tree cover, at very low speeds and during sharp acceleration. Treat any app reading as an estimate, never a legal measurement.
TL;DR
- Speed comes mainly from Doppler, not position differences.
- Open sky and steady speed give the best results.
- Tunnels, canyons and multipath cause errors.
- Low walking speeds look jumpy.
- Your car speedometer reads high by design.
How phones measure speed
A GNSS receiver tracks signals from GPS, Galileo, GLONASS and other constellations. It can compute speed by differencing positions, but most receivers also measure the Doppler shift of each satellite's carrier, which gives velocity directly and with less noise. Android and iOS then report a speed value with each location update; Android also reports a speed accuracy estimate. Read more in how GPS speedometers work.
What the official figures say
GPS.gov states that GPS-enabled smartphones are typically accurate to within about 4.9 m (16 ft) radius under open sky, and that accuracy worsens near buildings, bridges and trees. That is a position figure. Speed derived from Doppler is usually tighter at steady motion, but the exact number depends on the chipset, antenna and conditions, so we do not quote a single guarantee.
GPS speedometer accuracy killers
- Urban canyons: tall buildings block satellites and reflect signals (multipath), making speed spike.
- Tunnels and parking garages: no signal at all; the reading freezes or drops. See GPS in tunnels.
- Dense tree cover, and metallised (heat-reflective) windscreens that weaken signals inside some cars.
- Cold start: the first fix after opening the app can be rough for a few seconds.
- Hard acceleration or braking: filtered output lags the true change.
Slow speeds and walking
At walking pace the noise in each fix is a large share of the real speed, so the number can wobble between, say, 3 and 6 km/h. Averages over a minute are more trustworthy than the instant value. Our average walking speed guide has context.
Getting the best reading
Mount the phone where it sees the sky, such as low on the windscreen, keep location set to precise, and give it a few seconds after opening. Read improve GPS signal for more. Never adjust the phone while driving; set it up before you move.
GPS vs your dashboard
If your GPS app shows 57 mph while the dashboard shows 60, that is normal: regulations require car speedometers never to show less than true speed, so most read slightly high. See GPS vs car speedometer.
Related: GPS speedometer guide · How GPS speedometers work · GPS vs car speedometer: which one is right? · How to improve GPS signal · GPS in tunnels: why speed freezes · GPS speedometer: live speed from your phone's satellites · Digital speedometer: big numbers at a glance
Get Speedometer PRO on Google Play · Download on the App Store · Set it up before you move; never handle your phone while driving.
Sources
- GPS Accuracy (GPS.gov): gps.gov (accessed 26 October 2026)
- Doppler effect (Wikipedia): en.wikipedia.org (accessed 26 October 2026)
- Location and sensors (Android Developers): developer.android.com (accessed 26 October 2026)
- Location Services on iPhone (Apple Support): support.apple.com (accessed 26 October 2026)
- Multipath propagation (Wikipedia): en.wikipedia.org (accessed 26 October 2026)
- Urban canyon (Wikipedia): en.wikipedia.org (accessed 26 October 2026)
- UN Regulation No. 39: speedometer and odometer (UNECE): unece.org (accessed 26 October 2026)
FAQ
Is GPS speed more accurate than my car's speedometer?
At steady speed in open sky, often yes, because car speedometers are designed to read slightly high. In tunnels or cities the car's own reading is more dependable.
Why does my GPS speed jump around?
Signal reflections from buildings, partial blockage and low speeds all add noise. Values stabilise in open areas and at steady speeds.
Can a GPS speed reading be used to contest a ticket?
Do not rely on it. A phone app is not a calibrated measuring device, and courts treat evidence differently by jurisdiction. Ask a qualified lawyer.
Does more satellites mean better accuracy?
Generally, better geometry and more constellations help. Modern phones use GPS plus Galileo, GLONASS or BeiDou where available.
Is Speedometer PRO's speed exact?
No app is exact. Speedometer PRO shows the speed your phone's location system reports, which is an estimate affected by signal conditions.