How to Use Your iPhone as a GPS Speedometer (and How Accurate It Is)

TL;DR

SpeedTrace reads speed from the iPhone GPS, rejects stale or low-confidence fixes, lightly smooths the number and shows a status (Acquiring GPS, GPS Good, Weak GPS). It never fakes a confident reading on a weak signal. Needs a clear sky view.

The problem

Your car’s speedometer reads a few per cent high by design, a bike has none, and a boat or a plane needs speed over the ground, not wheel speed. A phone has the sensor, but most speedometer apps are cluttered with ads, show a number that jumps around, or confidently display 0 or a stale value when the GPS has lost the signal.

What you see

On the main screen:

  • A large, high-contrast speed on the dial you have chosen (Classic is free).
  • Altitude, compass heading, and session max and average speed.
  • A GPS status: Location Off, Acquiring GPS…, GPS Good or Weak GPS.
  • A live route map (see the trip guide) and the speed limit sign if you have turned that on.

Steps

  1. Open SpeedTrace and allow location access (“While Using” is enough to show speed).
  2. Wait for GPS Good. Outdoors this takes a few seconds.
  3. Choose your unit: km/h, mph, knots or m/s (see the units guide).
  4. In Settings → Display, turn on Keep Screen Awake if the phone is mounted.

How the number is made

  • Speed is taken from the GPS Doppler measurement, which is far better than differencing positions.
  • Fixes older than a few seconds (iOS sometimes hands back a cached one when you start) are rejected.
  • Low-confidence readings are held rather than shown; when needed the app falls back to speed worked out from positions.
  • The displayed value is lightly smoothed so the digits don’t jitter. Animate Speed Digits can be turned off in Settings.

When not to trust it

Tunnels, parking garages, urban canyons and dense tree cover. It also reacts a fraction of a second after you accelerate or brake. For legal or competitive timing use a certified device.

Battery

GPS uses power. Background location is only used while you record a trip.

FAQ

Does it need internet or cell signal? No. Speed comes from the GPS receiver. The map and optional speed-limit lookup need a connection; the speedometer does not.

How accurate is it? GPS speed from an iPhone is usually within about 1 km/h in good conditions, but it lags a little and jumps in tunnels, canyons or poor signal. It is not a calibrated or legal instrument.

Why does it say Weak GPS? The horizontal accuracy is poor, for example indoors, in a tunnel or between tall buildings. SpeedTrace shows that state rather than pretending.

Is it free? Yes, with the Classic dial. Other dials, HUD, 0–100 timing, colours and alternate icons are Pro or individual purchases.

The app SpeedTrace →