GPS systems

Hi Mick

Is the odometer out as well?

Its illegal to make a car with the speedo error you reported.

The speedo has to not read slow and can legally be up to 6% fast.

I had one Patrol once that was like this , its speedo said 107 when doing 100 on gps and this is actually pretty common.

I actually took mine out of the car and got a new one from the dealer which was also wrong by the same amount.
On mine there was a calibration adjustment (adjustment screw) and I re-calibrated it.

You probably need to ensure that your vehicle is tracking level and straight when comparing a GPS speed measurement against your speedo. I suspect that the mathematics in the GPS solution measure all acceleration and you are provided with a cumulative effect. I have noticed a difference in GPS reading against speedo (an increase) when following a curved path. This may also apply if climbing or descending at a constant speedo reading. The speedo is simply measuring wheel rotation and converting it to speed regardless of change in direction.

Another factor affecting GPS is the rounding due to inaccuracies of constant position updates. Even when stationary a GPS will show movement in various directions and velocities.

The elevation of the satellites above the horizon will affect accuracy of the GPS. If all the satellites are overhead the altitude solution will be more accurate but if the satellites are closer to the horizon the geographic position is more accurate.

On the subject of speedo accuracy I have a Prado that under-reads about two per cent and a Hilux that over-reads twelve per cent! Both vehicles are less than four years old. I raised the issue of the Hilux during a recent service and was advised that the error was not uncommon. Keeps me out of the radar traps anyway!