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My 2004 Prado with OEM rims and tyres has always at 100kph indicated read 2-3kph less than what each of my three GPS say. That may be because I run the tyre manufacturers recommended pressures and not what Toyota suggests. This increases the rolling diameter.
I only ever compare the speedo and GPS when it has a good fix and I've been traveling on a straightish section for several seconds. You need the GPS track points to fall on the road otherwise the GPS will read slow.
I hadn't heard of the method of getting the GPS speed for SWMBO's Passat. The Satnav system does not display on screen what it thinks the ground speed is. I could use the Garmin which does.
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