Garmin GPS speed accuracy

darkevil

darkevil

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Traveling on the M2 highway, cruising at 120km "as indicated on my clock "Ford Ranger 2.5 Turbo diesel in a 100km zone. My Garmin was showing a accurate speed of 107km per hour. Passing our ever present and very well hidden traffic officer manning a lazer gun i wondered if i am over the limit, or in for a nasty supprise at the end of the month.

Question is how admissible would a G.P.S reading be in our court of law?
 
They are almost spot one, we have tested one against car odometer (which varies depending on tire size ;) ) and against a speed camera, +-5km variation either side.
 
I have Garmin XP on my N95 and also usually have about 10km/h slower reflecting on there compared to my speedometer when doing over 100km/h.

I have always been of the understanding, perhaps incorrectly, that most car speedometers over indicate your actual speed. I assume car manufacturers would rather show you to be going faster than you actually are than vice versa!
 
How would tire size affect your speed reading? As far as I know your speed is measured by some moving part in your rngine and not your axle/tire...
 
the gps speed reading will be accurate, much more so than your speedometer.

but the accuracy will be lost when going downhill or uphill. uphill your gps could be showing 100km/h, when you are in fact doing 110km/h in real life, and a speedo showing 115km/h+

if it makes no sense to you, think of triangles..:-)

Just_Ice, the wheel diameter will significantly change the indicated reading.
one revolution of a wheel is not a constant, until you know the tyre size and profile etc.
 
Just_Ice said:
How would tire size affect your speed reading? As far as I know your speed is measured by some moving part in your rngine and not your axle/tire...
Click to expand...
Because, actual speed over ground is translated from the wheel axle revs on the gearbox so, without recalibration for the new tyre size, you're guaranteed to be off. Given that people usually put on bigger wheels, you end up with under-reading your speed but getting lower engine revs to do so ..but also getting lousier take off times too! Obviously smaller diameter tyres will do things in the opposite direction.
 
Its wrong.
I used a Garmin Nuvi, Sony wayfinder and N82, all the reading varied.
 
yeah, definitely some error involved. I used the GPS, speedometer and OBC. The speedo was always about 5-10km higher than the other 2 when going over 120km/hr.
 
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