How GPS Calculates Your Speed
GPS speed comes from measuring your position changes over time, not a radar gun. Here is how it actually works.
Position first, speed second
A GPS receiver does not measure speed directly. It calculates your position from signals sent by satellites, then works out speed by comparing how your position changes between fixes. Most modern devices also get a direct Doppler-shift speed reading from the satellite signal itself, which is usually more accurate than calculating speed from two separate position points.
Why readings can jitter
Speed readings can jump around for a few reasons: fewer visible satellites, signal reflecting off buildings (multipath), or being indoors or under heavy tree cover. None of these mean your phone is broken — they are normal limits of how GPS works.
What affects accuracy
- Open sky view vs. being surrounded by tall buildings or trees
- How many satellites your device can see
- Your device's GPS chipset quality
- Whether you are stationary or moving (standing still, GPS speed should read close to zero)
What this means in practice
Under open sky, modern phone GPS is typically accurate to within a few percent of true speed. It will rarely be perfect, but it is a reliable reference — not a toy.
Try it yourself
GPS Speed Test — free, no install, works in your browser.
Frequently asked questions
Is phone GPS speed as accurate as a car speedometer?
They measure different things and can differ by a few percent. Under open sky, phone GPS is usually a close, reliable reference.
Why does my GPS speed show 0 when I am moving?
This usually means a weak or lost signal — check that you have sky visibility and that location permission is granted.