How Accurate Is a GPS Speedometer?
Short answer: at a steady speed under open sky, within about 1 mph — which is very likely closer to the truth than the dial behind your steering wheel. Here's why, and where the caveats live.
Where the number comes from
GPS receivers don't compute speed by timing you between two map points. They measure the Doppler shift of each satellite's signal — the same physics that changes a siren's pitch as it passes — which yields your velocity directly. It's a per-second measurement of motion itself, which is why GPS speed holds rock-steady on cruise control instead of wandering. Speedometer Tracker requests iOS's best-for-navigation accuracy tier with no distance filtering, the highest fidelity the phone offers.
When to trust it
- Steady speeds, open sky: highway cruising, boating, cycling on open roads — this is GPS's home turf, and ~1 mph accuracy is typical.
- Watch the signal badge: the app shows live GPS signal quality. A solid fix means a trustworthy number; the indicator is your honesty meter.
- Checking your car's dial: the classic use case — dashboards legally must never read low, so they're calibrated high. The comparison guide walks through it.
When to doubt it
- Sky blockage: tunnels and parking garages kill the signal outright; urban canyons and heavy tree cover degrade it. The signal indicator will tell on them.
- Hard acceleration and braking: readings lag by a beat during rapid changes — GPS reports slightly smoothed reality.
- Very low speeds: below walking pace, noise dominates; no GPS speedometer is a good pedometer.
- The first seconds: give the receiver a moment after launch to acquire a solid fix before trusting the readout.
Practical calibration ritual
- Mount the phone with a clear sky view (dashboard beats footwell).
- Wait for a good signal badge, then reset the session stats.
- Hold a steady speed on a straight, open stretch and compare readings — dashboard vs GPS. The gap you see is your car's built-in optimism.
The one-sentence rule: steady speed + good signal = trust GPS; obstructed sky or rapid changes = trust it a beat later. The signal-quality badge exists so you never have to guess which regime you're in.
FAQ
How does GPS measure speed?
From satellite Doppler shift — direct velocity measurement, steady within ~1 mph under open sky.
When is GPS speed unreliable?
Blocked sky (tunnels, garages, urban canyons) and rapid speed changes — watch the signal badge.
Is GPS or my car's speedometer more accurate?
At steady speeds with good signal: GPS. Dashboards are calibrated to read high on purpose.