VaneGPS Speed vs Your Car's Speedometer: Why They Differ
If your phone says 96 and the car says 100, both are probably working as designed. Car speedometers are required never to read low, so most read a little high. GPS has no such bias.
Updated 1 October 2026
The short answer
In open sky at a steady speed, a phone's GPS speed is usually within about 1 km/h of your true speed. A car's speedometer is usually a few percent high, on purpose. So the GPS figure is the closer one. Because the speedometer's error is mostly a percentage, the gap grows with speed: a few km/h at town speeds, more on the motorway.
The rule that makes speedometers read high
In the EU, the UK and the many other countries that apply UN Regulation No. 39, a speedometer must never show less than the true speed. At the test speeds it may show more, up to 10% of the true speed plus 4 km/h:
0 ≤ indicated − true ≤ 0.1 × true + 4 km/h
| True speed | Allowed reading |
|---|---|
| 30 km/h | 30 to 37 km/h |
| 50 km/h | 50 to 59 km/h |
| 100 km/h | 100 to 114 km/h |
| 120 km/h | 120 to 136 km/h |
Because reading low is forbidden and wheels, tyres and sensors all vary, manufacturers aim slightly high so every car leaves the factory on the safe side. The test is done with the tyres the model normally comes with, inflated to the normal pressure. Production cars get a little more slack than the test car.
The United States has no federal speedometer accuracy standard. The old federal speedometer rule was withdrawn in 1982, and accuracy is left to manufacturers and voluntary industry practice. In practice US cars also tend to read slightly high.
Odometers are a separate story. UN R39's newest revision, in force since September 2025, adds a requirement that the distance shown be within ±4% of the true distance. Unlike speed, the odometer may read low as well as high.
Tyres change the reading
A car measures speed by counting wheel turns, so it assumes a fixed tyre size. Anything that changes the real rolling circumference changes the reading:
- Worn tyres are smaller. Each millimetre of tread lost shrinks the diameter by about 2 mm, roughly 0.3% on a typical car tyre. From new (about 8 mm of tread) to the legal minimum (1.6 mm in the UK and EU), the speedometer and odometer read about 1.5 to 2% higher than they did on new tyres.
- Larger wheels or tyres than standard make it read low, which can put a speedometer below the true speed. Lifted trucks and SUVs on oversized tyres are the classic case.
- Pressure changes the rolling radius slightly. Under-inflated tyres read a touch high.
When the GPS is the one that is wrong
GPS speed is very good in steady driving under open sky, and less good when:
- You are accelerating or braking hard: GPS updates about once a second, so it lags quick changes a little.
- The signal is poor: tall buildings, tree cover, or the phone tucked low in the car. Reflections can add noise or brief jumps.
- You are going up or down a steep hill: most phones report horizontal speed, so on a 10% grade the GPS reads about 0.5% low.
- You are in a tunnel: there is no signal at all. See why GPS stops in tunnels.
Which one should you drive by?
Your vehicle's speedometer. It is the instrument the law expects you to use, and because it reads high, it keeps you on the safe side. A phone is useful to check how far off your speedometer is, for example after changing tyre size, and as a speedometer where there is none, such as on a bicycle or a boat. No phone app is a legal speed reference.
Check your own car
- On a straight, flat, open road, hold a steady speed with cruise control if you have it.
- Have a passenger read the GPS speed. Our online speedometer works for this.
- Compare at two or three speeds. The difference is your speedometer's error. A constant percentage points to tyre size, and a constant offset is built in.
Sources
- UN Regulation No. 39 (speedometer and odometer), 01 series as published in the EU Official Journal, OJ L 2018/1857, paragraph 5.4 and Annex 3; 02 series with the odometer requirement, OJ 2025/1902.
- NHTSA interpretation noting the rescission of FMVSS 127 (speedometers and odometers) in 1982, AIAM 3608.
- US Government, GPS accuracy.
- Tyre-wear figures are our arithmetic from tyre dimensions, not a published standard.
Common questions
Why does my speedometer show a higher speed than GPS?
Car speedometers are built never to read below the true speed. Under UN Regulation 39, used in the EU, UK and many other countries, they may read up to 10% plus 4 km/h above it, so most read a few percent high. GPS has no such bias.
Is GPS speed accurate?
At a steady speed under open sky, phone GPS speed is typically within about 1 km/h. It is less reliable during hard acceleration, among tall buildings and in tunnels.
Can I use a phone app as a legal speed reference?
No. Drive by the posted limit and your vehicle's own speedometer. A phone is useful to check how far off your speedometer reads.