Speedometer V'S GPS

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Speedometer V'S GPS

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Brought a Nextbase Dashcam, (NBDVR322GW) about 5 weeks ago.. When I explored the manual further, you have the option of constantly having your speed shown on the Dashcam Screen.
It was really strange driving and having constantly updated speed showing.
After a few weeks driving, I started to wonder which is Most Accurate. Car's Speedometer or GPS?
When my Speedometer says 30mph, the GPs shows 28mph.
When I drive at 40mph, the GPS shows 38pmh.
Which is the right figure?
After a lot of thought, I decided to use my car's speedometer.
Why?
Well if I use car's speedometer I will always be on the right side of caution,
Anyone agree with me?
 
Brought a Nextbase Dashcam, (NBDVR322GW) about 5 weeks ago.. When I explored the manual further, you have the option of constantly having your speed shown on the Dashcam Screen.
It was really strange driving and having constantly updated speed showing.
After a few weeks driving, I started to wonder which is Most Accurate. Car's Speedometer or GPS?
When my Speedometer says 30mph, the GPs shows 28mph.
When I drive at 40mph, the GPS shows 38pmh.
Which is the right figure?
After a lot of thought, I decided to use my car's speedometer.
Why?
Well if I use car's speedometer I will always be on the right side of caution,
Anyone agree with me?
GPS will definitely be more accurate at least of a level road. Your cars speedo has to underread by law
 
GPS is dependent on timing a signal from several satellites

things get a bit squirrely when there isn't a clear line of sight to several the satellites especially around buildings and cliffs

A High percentage of the time the GPS will be bang on. But I wouldn't rely on it day to day.
 
GPS is dependent on timing a signal from several satellites

things get a bit squirrely when there isn't a clear line of sight to several the satellites especially around buildings and cliffs

A High percentage of the time the GPS will be bang on. But I wouldn't rely on it day to day.
 
Being a pedant, spedomeers don't HAVE to under read by law, but they can't over-read. The Regulations (EUN CE Regulation 39) say "not less than the actual speed" or up to 10% of reading +4 kph over-read.
They also say that the speedo must be independent of external resources i.e. you can't use a GPS for the mandated speedometer.

On GPS it CAN be very accurate for speed, but only under certain conditions. These are that the speed and direction are consant for at least 3 position updates. For normal GPS receivers the position update rate is once per second (1Hz) So if you re driving at constant speed (according to the speedo) in straight line for at least 3 seconds the GPS indicaed speed will be accurate (Normally better than +_ 0.1 MPH).The specalist GPS unit used for race cars etc have 10 Hz position upde rate so give better accuracy under dynamic conditions. Note that some cheap GPS modules cliam 10Hz but do not make full position updaes at this rate.
Yes I'm a bit of a GPS geek. I alo use them professionally. I first designed aircraft flight test equipmen using GPS modules in the early 90's. I considered GPS for the Speedometer i Thrust SSC but any GPS capable of > 600MPH was subject to American export restrictions. Thrust SSC eventually used two speed indicators. These werea MACH meter which was driven by wheel speed sensors and an airspeed indicator. The airspeed indicator was pnematically operated and required no power other than air pressure caused by the speed of the car. I designed and built both instruments. I actually own the MACH meter but it is on permanent loan and fitted to the car.

Robert G8RPI.
 
Phew. That's a lot to take in. GPS has been around a long time then. I am glad I got a Dashcam. If nothing else it gives me a visual record if I ever need it. Thanks.
 
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