Why it matters
Knowing which number to trust matters when the difference is a speeding fine, or when you’re timing a 0-100 run. Each source is accurate in a different way.
Car speedometers read high.
Rules in many countries forbid a speedometer from showing less than your true speed, so manufacturers set them to read a little over.
- A few percent high is normal, often more at higher speeds
- Worn tires and different tire sizes change it further
- The car’s speed over OBD2 comes from the same wheel sensors
How GPS measures speed.
Your iPhone works out speed from the satellite signal itself, which is very precise when the signal is good.
- Most accurate with a clear view of the sky
- Less reliable in tunnels, under bridges, between tall buildings and under heavy trees
- Updates about once a second, so very fast changes are smoothed
- No calibration needed, and tires make no difference
How Odo helps you judge.
Odo shows how good the signal is, and can show the car’s own reading next to it.
- GPS signal indicator on the speedometer and HUD, from Excellent to Weak
- With an OBD2 adapter, the car’s speed appears in live data to compare
- Recorded trips use GPS speed
Questions about GPS speedometer accuracy
Is GPS speed more accurate than my car’s speedometer?
Usually, yes, with a good signal. Car speedometers are set to read slightly high by design, while GPS measures your actual speed over the ground. In tunnels or dense city streets GPS can briefly be less precise.
Why does my speedometer app show less than my car?
Because the car’s speedometer reads a little high. A gap of a few km/h or mph at highway speed is normal. If the gap is large, check the GPS signal indicator and your tire size.
Does GPS speed work without internet?
Yes. GPS speed comes from satellites, not from mobile data. You only need data for maps and place names.
How accurate are 0-60 times measured with GPS?
Good enough to compare your own runs, as long as the signal is strong and the road is flat. Because the iPhone reports position about once a second, very short runs carry more error than long ones, so compare times from the same place in similar conditions.
How accurate is the iPhone GPS speedometer?
Very accurate with a clear view of the sky, and a signal indicator on the speedometer shows how good your GPS fix is. Full answer
Does Odo work without internet?
Yes for core tracking: GPS speed, distance and route recording work without a connection. Maps, navigation, Convoy, SOS and iCloud sync need one. Full answer
Works well with
Speedometer and HUD
50+ gauge designs, a mirrored windshield HUD, map mode and speed limit alerts.
Learn moreAcceleration timer
0–60 mph, 0–100 km/h and custom runs, timed automatically from your trips.
Learn moreOBD2 live data explained
What each engine reading means, what normal looks like, and why it matters.
Learn moreLast updated
