GPS vs Car Speedometer or Trusted Speed
Introduction
Long time ago I started to think what is more accurate in terms of velocity - GPS or Car Speedometer. Many people and me as well noticed, that car speedometer usually shows higher speed, than GPS devices. I wanted to make exact measurement and comparison of different speed control devices for a long time. Finally I did it and here is the description.
Scientific background
Officially GPS chip accuracy is 0.1 m/s (0.36 km/h) for velocity and from 1 to 10 meters for position according to technical specification for one of the chip (SiRF Star III) available by this link http://nz.apexelex.com/specs/modules_gps/MG-S02_v1.04.pdf and here as well
If we believe technical specification, GPS is very precise and official car’s speedometer precision I could not find from the first glance.
Experiment details
I took these devices for comparison:
I switched on all devices and drove near Police Radar. I recorded by video camera speed on all devices at the same time. To minimize errors I used cruise control to get stability in the speed. The results are shown on this video:
To compare devices on the higher speed I went to the highway with the limit 100 km/h and recorded all devices (except Police Radar :)) again. Recording is here:
Results and Conclusion
What I found as results and conclusion:
Long time ago I started to think what is more accurate in terms of velocity - GPS or Car Speedometer. Many people and me as well noticed, that car speedometer usually shows higher speed, than GPS devices. I wanted to make exact measurement and comparison of different speed control devices for a long time. Finally I did it and here is the description.
Scientific background
Officially GPS chip accuracy is 0.1 m/s (0.36 km/h) for velocity and from 1 to 10 meters for position according to technical specification for one of the chip (SiRF Star III) available by this link http://nz.apexelex.com/specs/modules_gps/MG-S02_v1.04.pdf and here as well
If we believe technical specification, GPS is very precise and official car’s speedometer precision I could not find from the first glance.
Experiment details
I took these devices for comparison:
- Car (Volkswagen Golf 6 Plus) with digital speedometer for better precision
- iPhone6 with GPS
- Garmin Fenix3 multisport watch with GPS
- Smartphone Sony Xperia S (LT26i is the technical name) with GPS
- Police Radar on the street with 50 km/h limit :)
I switched on all devices and drove near Police Radar. I recorded by video camera speed on all devices at the same time. To minimize errors I used cruise control to get stability in the speed. The results are shown on this video:
Click to viewTo compare devices on the higher speed I went to the highway with the limit 100 km/h and recorded all devices (except Police Radar :)) again. Recording is here:
Results and Conclusion
What I found as results and conclusion:
- First some numbers. City:
- GPS velocity and Police radar are exactly match each other. It was 46 km/h on all GPS devices and 46 km/h for Police Radar, but 50 km/h on car speedometer.
- Car speedometer shows higher speed than GPS devices and Police Radar.
- Seems car speedometer and GPS velocity meter has the constant difference around 8%. GPS speed 8% lower than Car Speedometer speed. Please do not consider this information as complete and final. I tried only one car and only one police radar, which is not really used for giving a fine, but just for giving a warning. Other police radar or other car speedometer can be different. If you will use only GPS speed most probably you will get a fine (ticket) from police. This post is for entertainment and information purposes ONLY!!! I am not responsible for anything that may happen from using information presented here. If you try to use only GPS device to check your speed it is your own risk.
- Car speedometer: 50 km/h
- Police Radar: 46 km/h
- GPS Smartphones (iPhone6 and Sony Xperia S): 46 km/h
- Garmin Fenix3: 46.5 km/h
Highway:
- Car speedometer: 100 km/h
- iPhone6: 93 km/h
- Sony Xperia S: 92 km/h