How accurate are GPS speedos

My RRc has a increasing accuracy as the speed increases. At 60KPH by the speedo the GPS (NavMan and Garmin) read 65KPH, at 80KPH by the speedo it's 83KPH by the GPS an all the readings correspond at 110KPH.

When you go through the radar speed checks on Victorian main roads, my NavMan is either spot on or +/- 1KPH which will be difference in averaging up or down of the 2 devices and the time delay between reading the sign and the GPS.

The result is that I trust my GPS' but the Law in NSW doesn't recognise them as accurate.

Diana

P.S. I find the NavMan very innaccurate in the M5-East tunnel! :D :D

Just an aside,

I always thought it was illegal - at least in Australia - for a speedo to read low. Eg read 100k when you are really doing 105k. But my Disco2 reads 110k when my GPS says 113k. And no it's not the GPS as I recently upgraded it and both the old and new read this.

No problem if you are aware of it, but just wondered if it's common in LR's?

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Originally Posted by e3j

Just an aside,

I always thought it was illegal - at least in Australia - for a speedo to read low. Eg read 100k when you are really doing 105k. But my Disco2 reads 110k when my GPS says 113k. And no it's not the GPS as I recently upgraded it and both the old and new read this.

No problem if you are aware of it, but just wondered if it's common in LR's?

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Putting larger tyres on will do that.

Quote:

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Originally Posted by e3j

No problem if you are aware of it, but just wondered if it's common in LR's?

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I have found the opposite. On both my IIA and 110, going from (factory) 32" diameter tyres to 33" tyres has made the speedo almost 100% accurate.

On a trip across the Nullarbor in the 110 we checked it against a GPS, and over 100 gps km my ODO showed 105.

While we are on this topic, how accurate are GPS units at measuring altitude these days??? (assuming clear sky above you)

Easily seen by putting the gps down on the beach.
Not very! Ten meters would be average on mine.
Needs birds overhead and on the horison to get a good calculation.
cheers

i have the mio 220 when it had the 2007 map on it , it was spot on with the speedo but now it has the 2008 map on it , it is 3 ks out

Nearly all car speedo's read 10% fast on std tyres