Radar guns, laser (LIDAR) guns and GPS all measure speed — but in completely different ways. Understanding how each works explains why their readings can differ slightly.
How a radar gun works
A radar gun sends out radio waves and measures the frequency shift of the waves bouncing back off a moving vehicle (the Doppler effect). That shift reveals the vehicle's speed relative to the gun.
- Accuracy: very good when used correctly, typically within 1–2 km/h.
- Limitation: it measures speed along the line between the gun and the vehicle, so an angle ("cosine error") can make the reading slightly lower than true speed.
How a laser (LIDAR) gun works
LIDAR fires rapid laser pulses and times how long they take to return, calculating distance change over time to get speed. It's more precise and targeted than radar.
How GPS measures speed
GPS uses the Doppler shift of satellite signals to work out how fast your position is changing over the ground — your true speed regardless of direction.
- Accuracy: typically within about 1 km/h in the open.
- Limitation: weak in tunnels, dense cities and under heavy cover.
Why the numbers can differ
- Reference frame: radar/LIDAR measure relative to the device and can under-read at an angle; GPS measures your actual ground speed.
- Your car's dashboard reads a few percent high on purpose, so it's usually higher than both.
- Signal conditions affect GPS momentarily.
In practice, on an open road GPS and a properly used radar gun agree closely — and both are typically closer to the truth than your dashboard.
Check it yourself
You can see your own GPS speed any time with the free GPS Speedometer — it reads your true speed over the ground in km/h, mph or knots, and runs entirely on your device. For how it compares to your car, see GPS vs car speedometer.