YOUR SPEED. YOUR DEVICE.
GPS Speed vs Car Speedometer: Why Readings Differ
A car speedometer is allowed to read higher than your true speed, but never lower. UN Regulation No. 39, used for vehicle approval in the EU and other countries that apply UN vehicle rules, permits the displayed speed to exceed true speed by up to 10% plus 4 km/h at its test speeds. GPS speed is calculated from satellite positions and is not required to lean either way, so a dashboard showing a few km/h more than your phone is normal.
XpeedThe rule behind the dashboard reading
Paragraph 5.3 of UN Regulation No. 39 says the speed indicated shall not be less than the true speed of the vehicle. At the test speeds, the displayed speed V1 and the true speed V2 must satisfy:
0 ≤ (V1 − V2) ≤ 0.1 V2 + 4 km/h
Worked through for two speeds:
- True speed 50 km/h: the dashboard may show anything from 50 to 59 km/h.
- True speed 100 km/h: the dashboard may show anything from 100 to 114 km/h.
The regulation also sets how the check is done. For a car with a top speed above 150 km/h, the test speeds are 40, 80 and 120 km/h. The equipment measuring true speed must be accurate to ±0.5%, and the tyres are set to the manufacturer's cold pressure plus 0.2 bar.
These figures come from Revision 1 of the regulation as published in the EU's Official Journal in 2010. Later amendments and other national rules can set different limits, and the version that applies depends on where and when the vehicle was approved.
Where GPS speed comes from
A browser speedometer asks the phone for its location. The browser can return a speed in meters per second, or no speed at all when the device has none to give. Xpeed uses that device speed when it is valid. When it is missing, Xpeed estimates speed from the distance between accepted positions over time and labels the value as estimated.
GPS.gov lists the signal's global average user range rate error as 0.006 m/s or less over any three-second interval. It describes the satellite signal. Your phone adds errors of its own. Accuracy at the receiver depends on satellite geometry, signal blockage, atmospheric conditions and receiver design. Buildings, bridges and trees block or reflect signals and can make the reading jump.
Why the two numbers drift apart
Several things affect the comparison besides the regulation's allowance:
- Timing. The dashboard and the phone update at different moments. While you accelerate or brake, one display can lag the other.
- Rounding. Both screens show whole numbers. A 1 km/h difference can come from rounding alone.
- Tyres. The regulation treats the tyre size designation as one of the features that define a vehicle type for speedometer approval. Tyres of a different size from the original fitment can therefore change what the dashboard shows. Ask the vehicle manufacturer or a qualified workshop before drawing a conclusion.
- Reception. A brief GPS dropout near tall buildings or under a bridge can produce a stale or estimated phone reading.
How to compare the two safely
The driver should never watch a phone while driving. Do the comparison with a passenger:
- Set the phone up before the trip, with both devices on the same unit.
- Choose a straight road with open sky, and hold a steady speed allowed on that road.
- The passenger reads both values at the same moment several times and notes them.
- Ignore readings taken while speeding up, slowing down or passing under structures.
- Compare the average difference, not a single pair of numbers.
Reading your result
A dashboard that reads somewhat higher than GPS at a steady speed matches what the regulation allows. Use the formula above to see whether the gap fits within 10% plus 4 km/h.
A dashboard that repeatedly reads lower than a steady open-sky GPS speed does not match the regulation's lower limit. That is a reason to have the vehicle checked by a qualified workshop. A phone reading alone does not prove a fault.
Xpeed is a browser tool, not a certified measuring instrument. It does not calibrate speedometers, and posted speed limits apply to the speed you drive, whatever either display says.
Questions and answers
Why does my car show 104 km/h when GPS shows 100 km/h?
At a true speed of 100 km/h, UN Regulation No. 39 allows the dashboard to show anything up to 114 km/h, but not below 100 km/h. A reading a few km/h high is within that allowance.
Is GPS speed always correct?
No. A phone's reading depends on reception and on the receiver inside the phone. Near buildings, bridges and trees, or while the phone is still finding satellites, the value can be wrong or missing. Xpeed shows a dash when no recent valid reading is available.
Can I use GPS speed to recalibrate my speedometer?
Xpeed does not provide calibration instructions. Speedometer adjustment and tyre changes belong with the vehicle manufacturer or a qualified workshop.
Does the 10% plus 4 km/h rule apply to every car?
It applies to vehicles approved under the UN Regulation No. 39 text quoted here. Other countries and later amendments can use different limits. Check the rules for your vehicle and country.