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Author Topic: Which is more accurate, speed reported by my GPS, or my speedometer?  (Read 13248 times)

Offline chris

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My GPS reports an average speed. Assuming I'm driving at a constant speed, is this a more accurate reflection of my car's velocity than the speedometer? And will this be influenced by hills etc?

Chris


 

Offline Soul Surfer

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The GPS reading should be much more accurate but can be a bit time delayed.  This can be used to calibrate your car speedometer.  (cruise control settings are very useful here) My personal experience over many years and many cars is that most car speedometers read between 2 MPH to 7 MPH fast being quite close around 30MPH and getting less accurate as you approach the legal limit of 70 MPH.  Bear in mind also that tyre wear can put in a small factor as well.

Below 30 MPH speedos can often read a bit slow but this is not usually critical.

Remember you can also use the 100meter posts and a stopwatch on a motorway to give an alternative independent calibration if you wish to confirm this.  It is also quite difficult to keep your speed constant manually to within 1MPH.
« Last Edit: 27/11/2010 10:40:48 by Soul Surfer »
 

SteveFish

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Chris, the GPS will read a little slow for ground speed when going up and down hills, so calibrate your speedometer on the flat.
 

Offline maffsolo

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Like asking which is worse bad or bad.
GPS reading is fine for distance of travel vs time it takes to get to the destination.
The GPS needs to have a ground map database programmed in which needs updating every so often.

Speedometer readings can vary relative to tire pressures/temperatures and tire size.

To calibrate the speedometer of the vehicle, a measured distance and time capture triggers are more reasonably accurate.
 Then there is the radar and laser guns that are used by law enforcement and sports training.

An external linear horizontal measurement from point to point vs elapse time is the most reliable.   
« Last Edit: 28/11/2010 00:05:03 by maffsolo »
 

SteveFish

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Maffsolo. Actually the GPS is very good. This is because it does both multiple time/distance calculations and Doppler shift measurements. Over half a kilometer, or half a mile, the accuracy should be more accurate than anything else available to ordinary folks. Steve
 

Offline maffsolo

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Maffsolo. Actually the GPS is very good. This is because it does both multiple time/distance calculations and Doppler shift measurements. Over half a kilometer, or half a mile, the accuracy should be more accurate than anything else available to ordinary folks. Steve

Only as good as the receivers updated software permits.
I experienced an on board GPS direction flaw, when it registered its shortest distance to my required  destination.
 This was an instance when it was sending me in the opposite direction
miles farther to get to a destination only a block away, I knew the area. Landmarks were crude but more efficient in this case.

This failure does not make me confident to use it as a precise device,
Since we as users can not tell without any real time feedback, how can you really trust it.
It is not always right... A device misinforms us once ....  I am more ordinary than most
« Last Edit: 28/11/2010 12:49:37 by maffsolo »
 

Offline Bored chemist

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"The GPS needs to have a ground map database programmed in which needs updating every so often"
Why?
It knows where I am now by relation to some orbiting clocks. It knows where I was earlier by the same means.
It knows the time difference to a silly degree of accuracy.
I'm not sure how well it copes with shifts in altitude, but it should be OK.
Of course, its job is easiest if the road is straight but even that shouldn't matter much.
It doesn't need a map because it knows where I am. As I move, it can draw its own map.
 

SteveFish

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Maffsolo, you are confusing bad map software with GPS. Also, the latest round of software that transcribes maps apparently has a bad algorithm or something. Some of the roads in my area are shown as pretty bizarre. Steve
 

Offline maffsolo

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Maffsolo, you are confusing bad map software with GPS. Also, the latest round of software that transcribes maps apparently has a bad algorithm or something. Some of the roads in my area are shown as pretty bizarre. Steve

Yes  you all are right that is what I was confused with.
 I had a line of site, a visual on my destination point, and the GPS directed me away to the longer route, way the heck around about way, to come up to the other side of my destination point.

The prime reason of the unit is to aid you in direction and travel, second is average speed, ironic that the second functionality is more precise. 
Thank guys
« Last Edit: 28/11/2010 22:47:28 by maffsolo »
 

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