speedometer

 

my car speedometer and my garmin speedometer are diffecurent. which one is more accurate?

See any of these threads

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------------------------ Phil Hornby, Stockport, England ----------------------               http://GeePeeEx.com - Garmin POI Creation made easy           »      

Here's one thing to also

Here's one thing to also keep in mind. Your speedo works from an analog signal: the turning of your transmission or wheels. Your GPS determines "speed" by measuring the distance between readings versus the time between readings.

Assume your GPS is accurate to 15 ft, which it will be sometimes, but not always. And assume your unit is updating once per second (this may vary from unit to unit) If you're standing still, and on one update the unit "thinks" you're at A, and on the next update the unit "thinks" you've moved 15 ft to B (because of inaccuracies) you will have travelled 15 ft in 1 sec, or just over 10 miles per hour. All while standing still smile

Also keep in mind that this "inaccuracy" is a 15 ft circle from A, and that exposes the possibility of your speed being -10 mph surprised

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nüvi 200

Speedo

my Garmin 760 has always had "dead on" accuracy. It does lag by about a second though. The accuracy has persisted with the few vehichles tested. Impressive.
smile

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nuvi' 2450

remember

If you start at 15 ft inaccuracy and then in 1 sec you travel 15 ft and you still have 15 ft inaccuracy then the unit is still accurate in calculating the speed.

If you start at 15 ft inaccuracy and then in 1 sec you travel 15 ft and you then have 10 ft inaccuracy then the unit would have an inaccurate calculation of your speed... or do you? What if the inaccuracy is to the north and you are traveling west, what then? confused

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Allan Barnett - Garmin nüvi 885T/765T/Pharos GPS (bluetooth) w/MS Maps on PPC

Circular innaccuracy

PCPro wrote:

If you start at 15 ft inaccuracy and then in 1 sec you travel 15 ft and you still have 15 ft inaccuracy then the unit is still accurate in calculating the speed.

As long as the 15 ft inaccuracy is in the same direction. Otherwise, see below.

PCPro wrote:

If you start at 15 ft inaccuracy and then in 1 sec you travel 15 ft and you then have 10 ft inaccuracy then the unit would have an inaccurate calculation of your speed... or do you? What if the inaccuracy is to the north and you are traveling west, what then? confused

The inaccuracy component is circular, therefore it could sense an inaccurate movement in any direction, with a corresponding speed/heading indication. I'd be willing to bet the software has some "fuzzy" logic around that calculation just to smooth that out.

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nüvi 200

movement

Quote:

could sense an inaccurate movement in any direction, with a corresponding speed/heading indication

This is why many see movement while sitting still. smile

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Allan Barnett - Garmin nüvi 885T/765T/Pharos GPS (bluetooth) w/MS Maps on PPC