Quick Answer: Under open sky and at a steady speed, a modern phone’s GPS speed is usually accurate to within about 1–2 km/h (1 mph). Accuracy drops in tunnels, between tall buildings, under heavy tree cover, during hard acceleration, and in the first seconds before the GPS lock settles. A clear view of the sky is the single biggest factor.
What Affects Phone GPS Speed Accuracy
| Condition | Effect on speed |
|---|---|
| Open road, steady speed | Best case: about ±1–2 km/h |
| Tall buildings (“urban canyon”) | Reflected signals cause jumps and spikes |
| Tunnels, garages, underground | No signal — speed drops to zero or freezes |
| Dense forest, heavy rain | Weaker signals, noisier readings |
| Hard acceleration or braking | Lags by about a second |
| Very slow movement (walking) | Noise is a bigger share of the reading |
| Phone deep in a bag or pocket | Weaker signal than a phone on a mount |
How to Get the Most Accurate Reading
- Turn on precise location (iPhone) or Google Location Accuracy (Android).
- Mount the phone where it sees the sky — dashboard, handlebars or an outer pocket.
- Wait 10–30 seconds after starting for the lock to settle; the accuracy radius shown on the speedometer should shrink.
- Compare speeds only when moving steadily.
- Turn off battery-saver modes, which can reduce GPS update rates.
Why Speed Is More Accurate Than Position
Phones calculate speed from the Doppler shift of satellite signals rather than from the change in position. That’s why speed can be accurate to about 1 km/h even when your position is only good to several meters. Read more in how a GPS speedometer works and GPS vs. speedometer accuracy.
Frequently Asked Questions
Is phone GPS speed accurate enough to test my speedometer?
Yes, at a steady speed on an open road. Use the speedometer accuracy check.
Are iPhones or Android phones more accurate?
Both use similar multi-constellation GPS chips. Differences between individual phone models and how they’re mounted matter more than the brand.
Why does my GPS speed jump when I’m stopped?
Position noise of a few meters can look like movement. Good speedometers filter very low speeds to zero when you’re stationary.