You're driving at a steady 100 on the dashboard, glance at a GPS speedometer, and it says 94. Which one is right? This is one of the most common questions drivers ask — and the answer is reassuringly simple.
Car speedometers are designed to read a little high
By law in most countries, a car's speedometer must never show less than your true speed — but it's allowed to show more. So manufacturers deliberately calibrate them to read slightly high, giving themselves a safety margin. A typical car shows about 2–10% more than your actual speed.
That's why, at a real 94 km/h, your dashboard might read 100. The car isn't broken — it's doing exactly what it's supposed to.
GPS measures your true speed over the ground
A GPS speedometer works differently. It calculates how fast your position is actually changing using satellite signals — the same method sat-navs use. This is your real speed over the ground, independent of your tyres, wheel size or the car's calibration.
So in most cases, GPS is closer to your true speed, and your car's dashboard is the one reading high.
Why the car can be "wrong" even when it's fine
A few things nudge a car's speedometer off:
- Tyre wear and size. Speedometers assume a standard tyre diameter. Worn tyres are slightly smaller, changing the reading. Fitting a different tyre size changes it too.
- The legal safety margin built in by the manufacturer.
- Tyre pressure, which slightly alters the rolling diameter.
When GPS can be off instead
GPS isn't perfect either. It can briefly read wrong when:
- You're in a tunnel, underground car park or dense city where the signal bounces or drops.
- You've just started and it hasn't locked on yet.
- You're surrounded by tall buildings ("urban canyon" effect).
In open conditions with a clear sky, though, GPS speed is very accurate — usually within about 1 km/h.
Check it yourself
Next time you're on an open road (as a passenger, or safely):
- Open the GPS Speedometer and tap Start.
- Hold a steady speed on the dashboard.
- Compare the two numbers.
You'll almost certainly see the GPS reading a few km/h lower than the dashboard — that gap is your speedometer's built-in safety margin.
The bottom line
Your car's speedometer reads slightly high on purpose and by law. A GPS speedometer shows your true speed over the ground. Neither is "broken" — but if you want to know how fast you're really going, GPS is the one to trust in open conditions. Try it free on the speedometer tool; nothing is uploaded and your location stays on your device.