Which is correct..odometer or GPS

**vnx205** · 15th July 2008, 05:43 PM

Results of steady state ground speed analysis revealed that the average error was less than 1 km h-1 (less than 5.3%), except for the 15 km h-1 data set where the average error reached 1.72 km h-1 (9.92%

So does that mean that the best result for the GPS is about twice the discrepancy that Stevo68 noticed between the odometer and the GPS?

I realise this test was speed and Stevo68 was measuring distance, but it does make you wonder about the GPS.

I was a bit surprised that Stevo68's difference was as small as 2% and even more surprised at the fact that the odometer gave the smaller figure.

I would have expected, as JDNSW suggested, to learn that it was about 8% and in the other direction. I have standard tyres on my Defender and I think the speed is out by 10% at 100kmh and the distance about 6% out.

**Utemad** · 15th July 2008, 05:52 PM

Going by their graphs and conclusion it appears that the error in GPS speed is around 1.2km/h (they say 1.27km/h) regardless of vehicle speed.
So 1.2km/h at 25km/h and 1.2km/h at 50km/h. So not the same sort of error as a speedo.

**101RRS** · 15th July 2008, 06:03 PM

Originally Posted by stevo68

then I realised that it had searched for about a k or 2 due to heavy cloud covering.

Then you must have an old GPS because this is not an issue for modern GPS systems.

Garry

RonMcGr · 15th July 2008, 06:21 PM

GPS of course.
Satellite, accurate as a slide rule!
I have never seen an accurate speedo or odometer in my life!
Mind you, our 1925 Citroen Cloverleaf was not too bad, it went so slow you could measure the road

**stevo68** · 15th July 2008, 06:40 PM

Quote:
Originally Posted by stevo68

then I realised that it had searched for about a k or 2 due to heavy cloud covering.

Then you must have an old GPS because this is not an issue for modern GPS systems.

Garry

Nope its a modern Garmin, though more likely a km of travelling. When it storming up here in the hinterland our Austar also gets affected,

Regards

Stevo

**waynep** · 15th July 2008, 07:04 PM

I'd swear in court by a spot speed measurement from GPS, ( which uses a rubidium clock ) but not a distance measurement. The reason being you can never be 100% certain your GPS was locked on a fix for the whole time.
So I do it this way - measure the difference in percentage ( plus or minus) between spot GPS speed and car speedo - use the car odometer for distance measurements but factor same percentage difference into the odometer reading.

**victa125** · 15th July 2008, 08:26 PM

Seems to me Stevo that both have variables, Tyre wear, pressures, wheel speed, etc, would affect the odometer which if ever had been calibrated properly, less than the actual speedo. The hilly thing reception etc means more calculations for the GPS. Should have no effect on odo, but what happens if we chuck heaps of corners in, the odometer can only work on a mean average of actual wheel distance traveled. Without any slippage that is. should both be exactly the same I would be more concerned.

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