copilot/ navman gps speed readings..how accurate are they?

I suspected the speedo was out as everyone sits up my clacker but I just want to be sure.

Just because the Speed is out doesn't mean the odometer is out - they are different calibrations - even for the old cable-driven ones.

I usually find the speedo indicates high, but the odometer is accurate

I am not sure about that Mike, you might be right.

But, why would they make one reading accurate, and the other reading so far out?

Is it a technology thing that they haven't been able to get right, or could there be some other reason?

Dave

Mike, as I understand it electronic speedos work by counting the number of tailshaft revolutions and are calibrated taking into account tyre diameter and diff ratio. X number of pulses = X numbers of km. Speed is derived from this distance vs time which is taken from an internal clock. There should therefore be no differing accuracy between the speed indicated and the distance indicated unless the manufacturer chooses to inflate the speed indication for reasons of safety factor in speed zones or to artificially inflate performance.
I have never checked the distance readings between a GPS and the speedo but it could be interesting to see the result given the speed readings vary considerably.
Ken

Noticed a difference on the Paj between the speedo and the odometer.

I have bigger tyres than stock by +10%.

Speedo now reads 98 at 100 true.
Odometer now under reads by 5%, i.e. 95km on the dial is 100k true.

So there is a 3% difference between speedo and odometer.

Checked by GPS a number of times over the years.

Difference could be due to the two sizes of tyres supplied in the Paj range at the time. 235/75/15 (29") and 265/70/15 (31"), so speedo and odometer set as a compromise towards the larger tyre sold on the more common GLS model. All still within the specs of the ADR.

They are usually seperately calibrated Dave, as the odometer is required to be more accurate and maintain its accuracy regardless of speed.