Written by Muneeb Rana · Last updated 26 October 2026
Quick answer: GPS speedometer vs radar: a GPS speedometer and a radar gun measure speed differently. Radar bounces microwaves off a vehicle and reads the Doppler shift of the echo; GPS speedometers read Doppler shifts in satellite signals received by your phone. Both use Doppler, but radar is a calibrated enforcement device while a phone app is an uncalibrated estimate.
TL;DR
- Radar: reflected microwaves.
- GPS: satellite signal Doppler.
- Radar devices are calibrated and tested.
- Phone readings are estimates.
- Don't rely on an app to contest a ticket.
Gps speedometer vs radar: how radar measures speed
A radar gun transmits a known frequency; the reflection from a moving car returns shifted in proportion to speed along the beam. Angles reduce the measured value (cosine effect). Wikipedia's radar gun article covers the physics.
Speedometer radar comparison
| Radar gun | GPS speedometer | |
|---|---|---|
| Signal | Reflected microwave | Satellite broadcasts |
| Who measures | Observer | You |
| Calibration | Tested per procedures | None |
| Weak spots | Angle, beam spread | Tunnels, cities |
Why readings differ
Your car's speedometer reads high; GPS lags in rapid changes; radar measures one instant along one line. Expect small differences.
Lidar and average-speed cameras
Lidar uses laser pulses timed over distance. Average-speed cameras time a vehicle between two points; see average speed vs max speed.
Legal reality
Phone readings are not calibrated evidence. If you receive a ticket, seek legal advice; do not rely on an app screenshot.
Cosine effect
A radar gun reads full speed only when pointed straight along the vehicle's path. At an angle, it reads lower by the cosine of the angle, so a 10-degree angle reads about 1.5% low. This works in the driver's favour, unlike speedometer error.
Why they disagree with your phone
Radar measures an instant; GPS speed is smoothed over a short window and depends on satellite geometry. A difference of 1–2 mph is unremarkable.
Which reading should you trust?
For your own driving, the vehicle's speedometer is the instrument the law expects you to use, and it is designed never to under-read. A GPS app is a useful cross-check: if it shows a consistently lower figure than your dashboard on a straight, open road, that difference is probably your speedometer's built-in over-read. Radar and lidar used by police are calibrated devices with their own testing procedures, and a phone app reading is unlikely to carry weight as evidence against them. If you believe an enforcement reading was wrong, take legal advice rather than relying on an app log. Treat every tool as an estimate, and the safest approach is simply to stay comfortably below the limit so small differences don't matter.
Our position
Speedometer PRO is for information only. It does not detect radar or cameras.
Related: GPS speedometer guide · How GPS speedometers work · GPS vs car speedometer: which one is right? · Average speed vs max speed · The dangers of speeding · Speedometer app safety: how to use it without distraction · Digital speedometer: big numbers at a glance · Analog speedometer: a classic needle dial
Get Speedometer PRO on Google Play · Download on the App Store · Set it up before you move; never handle your phone while driving.
Sources
- Radar speed gun (Wikipedia): en.wikipedia.org (accessed 26 October 2026)
- Doppler effect (Wikipedia): en.wikipedia.org (accessed 26 October 2026)
- Average speed camera (Wikipedia): en.wikipedia.org (accessed 26 October 2026)
- GPS Accuracy (GPS.gov): gps.gov (accessed 26 October 2026)
FAQ
Is GPS more accurate than radar?
They do different jobs. Radar devices are tested and calibrated for enforcement; phone GPS is an uncalibrated estimate.
Can I use my GPS app to fight a speeding ticket?
Do not rely on it. Courts usually treat calibrated enforcement devices differently. Get legal advice.
Does Speedometer PRO detect radar?
No. It shows your GPS speed only.
Why did radar read higher than my GPS?
Timing, angle, GPS smoothing and which vehicle the beam hit can all cause differences.
What is the cosine effect?
When radar is aimed at an angle, it measures only the component of speed toward it, which reads lower than true speed.